Deputy Chief Constable Pippa Mills said identifying the victims "remains our number one priority".
She appealed for anyone with information about the route the lorry may have taken, or anyone who had seen the vehicle or knew about its journey, to get in touch.
Prime Minister Boris Johnson said he was "appalled by this tragic incident".
He said: "I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened. My thoughts are with all those who lost their lives and their loved ones."
Home Secretary Priti Patel said she was "shocked and saddened by this utterly tragic incident", while Thurrock MP Jackie Doyle-Price said it was "sickening news".
"People trafficking is a vile and dangerous business," Ms Doyle-Price tweeted, adding that she hoped Essex Police could "bring these murderers to justice".
Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said if the lorry had come from Bulgaria, getting into Britain via Holyhead was an "unorthodox route".
He said: "People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.
"It's a long way around and it'll add an extra day to the journey."
The National Crime Agency said it had sent officers to assist the investigation. Ch Supt Andrew Mariner, from Essex Police, said he expected identifying the victims would be a "lengthy process".
Glen Freeland, from GSF Car Parts, which is close to where the bodies were found, said staff could not access their premises on Eastern Avenue.
"The manager went to get into work this morning and it was cordoned off and we've been moved to a different area," Mr Freeland said.
In June 2000, 58 Chinese immigrants were found suffocated to death in the back of a lorry in Dover. There were two survivors.
A Dutch lorry driver was jailed the following year for manslaughter.
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UK authorities were called to the scene in southeast England shortly before 1:40 a.m. on Wednesday morning, where they found the bodies of 39 individuals inside a truck.
Part of the line of inquiry for UK authorities is working out where the truck came from and how it ended up in an industrial park east of London.
Local police have said that they believe the truck originated from Bulgaria and that it entered Britain through the Welsh port of Holyhead over the weekend. A regular ferry service connects Holyhead with the Irish capital of Dublin.
"If the lorry came from Bulgaria, getting into Britain via Holyhead is an unorthodox route,” Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association (FTA) told PA news agency.
"People have been saying that security and checks have been increased a places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.
"It's a long way around and it'll add an extra day to the journey,” Leheny added.
LONDON — Police in southeastern England said that 39 people were found dead Wednesday inside a truck container believed to have come from Bulgaria.
The truck, which is said to have entered the country on Saturday, was found by ambulance workers at Waterglade Industrial Park, Grays.
“This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened,” Chief Superintendent Andrew Mariner said. “We are in the process of identifying the victims, however I anticipate that this could be a lengthy process.”
A cordon has been put in place and access to and from the Waterglade Industrial Park remains closed.
“We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally.” Mariner said.
When did this happen?
Emergency services rushed to Waterglade Industrial Park in Grays, at roughly 1:40 a.m. local time.
Have the bodies been identified?
All 39 people found were pronounced dead at the scene. Early indications suggest 38 are adults and one is a teenager.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," Mariner said.
Is there a suspect?
A 25-year-old-man from Northern Ireland who is the truck's driver has been arrested on suspicion of murder. He remains in custody.
Home Secretary Priti Patel tweeted: “Shocked & saddened by this utterly tragic incident in Grays. Essex Police has arrested an individual and we must give them the space to conduct their investigations.”
Boris Johnson will urge MPs to back his Brexit deal in a final bid to get the UK to leave the EU in nine days' time.
MPs will vote on the PM's Withdrawal Agreement Bill and if they back it they will be asked to approve a three-day timetable to consider the legislation.
But the decision to curtail the scrutiny of the bill to three days has sparked anger from opposition MPs.
The BBC's Norman Smith said the PM looks set for a "deeply damaging" defeat over the timetable.
However, the prime minister seems on course to win an important symbolic victory with MPs poised to back his Brexit bill in principle, our assistant political editor added.
MPs will begin a number of votes on Mr Johnson's Withdrawal Agreement Bill, which was published on Monday, at about 19:00 BST.
Elsewhere, European Council President Donald Tusk has told the European Parliament in Strasbourg he is consulting the 27 EU leaders on how to react to the UK's request for a Brexit delay and "will decide in the coming days".
"I have no doubt that we should treat the British request for extension in all seriousness," he tweeted.
MPs believed by ministers to be ready to back the prime minister include pro-Leave Labour rebels and former Tory MPs now sitting as independents who would rather leave with Mr Johnson's deal than no deal at all.
If MPs back the bill, they will then vote on the government's "programme motion", which sets out the timetable for the legislation's passage through the Commons.
On Monday, Leader of the Commons Jacob Rees-Mogg said: "People who do not vote for the programme motion will not be voting for Brexit on October 31."
Here it is. And here we go. The government has now published the pages and pages of new laws that need to be passed to make our departure from the EU happen.
Forget the meaningful vote, to get Brexit done - as the prime minister never tires of saying - this whole bundle of legislation has to pass.
The European Union (Withdrawal Agreement) Bill is a document of 110 pages that details exactly how Parliament is expected to put the deal that the prime minister agreed with his counterparts around the continent into UK law.
If the programme motion is approved, the bill will then move to the committee stage - which will continue on into Wednesday - when MPs will have the opportunity to put down amendments.
These are expected to include attempts to keep the UK more closely aligned with the EU through a customs union and to stage a second referendum.
Both are bitterly opposed by the government, raising the possibility that it could pull the bill altogether if either gets through.
Ahead of the debate, Mr Johnson said: "The public doesn't want any more delays, neither do other European leaders and neither do I.
"Let's get Brexit done on 31 October and move on."
However, Labour's shadow Brexit secretary, Sir Keir Starmer, accused ministers of trying to "bounce" MPs into approving a bill that could cause "huge damage" to the country.
What is the proposed timetable for the Withdrawal Agreement Bill?
Tuesday - second reading - MPs' first chance to debate the bill and vote for its continued passage. If passed at second reading, committee stage begins the same day.
Wednesday and Thursday - committee stage - where detailed examination of the bill takes place and specific amendments - on a fresh referendum, for example - can be tabled and voted on. The bill then moves on to report stage, which offers further opportunities for amendments before it moves to third reading. This is MPs' final chance to debate the bill before voting on whether to approve it.
If approved, it then moves to the Lords to begin a similar scrutiny process.
Shadow foreign secretary Emily Thornberry said Labour was "outraged" by the government's attempt to push the bill through in a short time.
"When I did the Health and Social Care Act, which was a major piece of legislation, it took three months," the Labour MP told BBC Breakfast.
"In order for politicians to do their job properly, we do need to have time," she added.
The SNP's Pete Wishart also condemned a lack of economic impact assessments of the deal ahead of the attempt to pass the legislation.
Ex-Tory MP Rory Stewart, who lost the party whip when he voted in support of the Benn Act, said he would back Mr Johnson's bill in principle but wanted to ensure Parliament had a "normal" amount of time to debate it.
"We need to finish this in the proper way, and if we're going to deliver Brexit, we need to deliver it in a way that Brexiteers and Remainers believe was taken through Parliament fairly," he told BBC Radio 4's Today programme.
But Mr Rees-Mogg said other acts had been brought and passed with short notice.
"A king emperor left in 24 hours, and we are removing an imperial yoke in over a week," he added.
And Housing Secretary Robert Jenrick told BBC Breakfast that Parliament "can move pretty quickly" and has already had 500 hours of debate on Brexit.
"So most MPs, frankly, are very familiar with the vast majority of issues that are in this bill," he said.
What is in the Withdrawal Agreement Bill?
The 110-page document will give legal effect to the withdrawal deal negotiated by Mr Johnson.
His plan ditches the backstop - the controversial measure designed to prevent a return to physical checks on the Irish border.
Instead it essentially draws a new customs border in the Irish Sea, as goods which could travel onwards to Ireland will have to pay a duty tax.
It also will see the whole of the UK leave the EU customs union, meaning it could strike trade deals with other countries in the future.
The WAB will also turn any agreed transition period into law, fulfil requirements on the rights of EU citizens in the UK after Brexit, and allow ministers to make "divorce payments" to the EU foreseen under the current deal.
But MPs will be able to vote on amendments - changes or add-ons - to the bill.
If the government cannot get the bill through Parliament, the default legal position is for the UK to leave without a deal on 31 October, but that will change if the EU grants an extension.
MPs instead backed an amendment withholding their approval until the bill has faced the scrutiny of both the Commons and the Lords and been passed into law.
The amendment worked alongside the so-called Benn Act, which required the prime minister to ask the EU for an extension to the 31 October deadline to prevent a no-deal Brexit.
Following Saturday's developments, Mr Johnson sent an unsigned letter asking for the delay, along with a signed letter saying why he did not agree with any further delays.
There has been no official word from the EU yet on whether the bloc will grant a delay or what length it would be.
Mr Tusk said the result of his consultation with EU leaders "will very much depend on what the British parliament decides or doesn't decide".
Do you have any questions about the Brexit bill and what happens next?
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If you are reading this page on the BBC News app, you will need to visit the mobile version of the BBC website to submit your question on this topic.
Boris Johnson will urge MPs to back his Brexit deal in a final bid to get the UK to leave the EU in nine days' time.
MPs will vote on the PM's Withdrawal Agreement Bill and if they back it they will be asked to approve a three-day timetable to consider the legislation.
But the decision to curtail the scrutiny of the bill to three days has sparked anger from opposition MPs.
The BBC's Norman Smith said the PM looks set for a "deeply damaging" defeat over the timetable.
However, the prime minister seems on course to win an important symbolic victory with MPs poised to back his Brexit bill in principle, our assistant political editor added.
MPs will begin a number of votes on Mr Johnson's Withdrawal Agreement Bill, which was published on Monday, at about 19:00 BST.
Elsewhere, European Council President Donald Tusk has told the European Parliament in Strasbourg he is consulting the 27 EU leaders on how to react to the UK's request for a Brexit delay and "will decide in the coming days".
MPs believed by ministers to be ready to back the prime minister include pro-Leave Labour rebels and former Tory MPs now sitting as independents who would rather leave with Mr Johnson's deal than no deal at all.
If MPs back the bill, they will then vote on the government's "programme motion", which sets out the timetable for the legislation's passage through the Commons.
On Monday, Leader of the Commons Jacob Rees-Mogg said: "People who do not vote for the programme motion will not be voting for Brexit on October 31."
Here it is. And here we go. The government has now published the pages and pages of new laws that need to be passed to make our departure from the EU happen.
Forget the meaningful vote, to get Brexit done - as the prime minister never tires of saying - this whole bundle of legislation has to pass.
The European Union (Withdrawal Agreement) Bill is a document of 110 pages that details exactly how Parliament is expected to put the deal that the prime minister agreed with his counterparts around the continent into UK law.
If the programme motion is approved, the bill will then move to the committee stage - which will continue on into Wednesday - when MPs will have the opportunity to put down amendments.
These are expected to include attempts to keep the UK more closely aligned with the EU through a customs union and to stage a second referendum.
Both are bitterly opposed by the government, raising the possibility that it could pull the bill altogether if either gets through.
Ahead of the debate, Mr Johnson said: "The public doesn't want any more delays, neither do other European leaders and neither do I.
"Let's get Brexit done on 31 October and move on."
However, Labour's shadow Brexit secretary, Sir Keir Starmer, accused ministers of trying to "bounce" MPs into approving a bill that could cause "huge damage" to the country.
What is the proposed timetable for the Withdrawal Agreement Bill?
Tuesday - second reading - MPs' first chance to debate the bill and vote for its continued passage. If passed at second reading, committee stage begins the same day.
Wednesday and Thursday - committee stage - where detailed examination of the bill takes place and specific amendments - on a fresh referendum, for example - can be tabled and voted on. The bill then moves on to report stage, which offers further opportunities for amendments before it moves to third reading. This is MPs' final chance to debate the bill before voting on whether to approve it.
If approved, it then moves to the Lords to begin a similar scrutiny process.
Shadow foreign secretary Emily Thornberry said Labour was "outraged" by the government's attempt to push the bill through in a short time.
"When I did the Health and Social Care Act, which was a major piece of legislation, it took three months," the Labour MP told BBC Breakfast.
"In order for politicians to do their job properly, we do need to have time," she added.
The SNP's Pete Wishart also condemned a lack of economic impact assessments of the deal ahead of the attempt to pass the legislation.
Ex-Tory MP Rory Stewart, who lost the party whip when he voted in support of the Benn Act, said he would back Mr Johnson's bill in principle but wanted to ensure Parliament had a "normal" amount of time to debate it.
"We need to finish this in the proper way, and if we're going to deliver Brexit, we need to deliver it in a way that Brexiteers and Remainers believe was taken through Parliament fairly," he told BBC Radio 4's Today programme.
But Mr Rees-Mogg said other acts had been brought and passed with short notice.
"A king emperor left in 24 hours, and we are removing an imperial yoke in over a week," he added.
And Housing Secretary Robert Jenrick told BBC Breakfast that Parliament "can move pretty quickly" and has already had 500 hours of debate on Brexit.
"So most MPs, frankly, are very familiar with the vast majority of issues that are in this bill," he said.
What is in the Withdrawal Agreement Bill?
The 110-page document will give legal effect to the withdrawal deal negotiated by Mr Johnson.
His plan ditches the backstop - the controversial measure designed to prevent a return to physical checks on the Irish border.
Instead it essentially draws a new customs border in the Irish Sea, as goods which could travel onwards to Ireland will have to pay a duty tax.
It also will see the whole of the UK leave the EU customs union, meaning it could strike trade deals with other countries in the future.
The WAB will also turn any agreed transition period into law, fulfil requirements on the rights of EU citizens in the UK after Brexit, and allow ministers to make "divorce payments" to the EU foreseen under the current deal.
But MPs will be able to vote on amendments - changes or add-ons - to the bill.
If the government cannot get the bill through Parliament, the default legal position is for the UK to leave without a deal on 31 October, but that will change if the EU grants an extension.
MPs instead backed an amendment withholding their approval until the bill has faced the scrutiny of both the Commons and the Lords and been passed into law.
The amendment worked alongside the so-called Benn Act, which required the prime minister to ask the EU for an extension to the 31 October deadline to prevent a no-deal Brexit.
Following Saturday's developments, Mr Johnson sent an unsigned letter asking for the delay, along with a signed letter saying why he did not agree with any further delays.
There has been no official word from the EU yet on whether the bloc will grant a delay or what length it would be.
Mr Tusk said the result of his consultation with EU leaders "will very much depend on what the British parliament decides or doesn't decide".
Do you have any questions about the Brexit bill and what happens next?
Use this form to ask your question:
In some cases your question will be published, displaying your name and location as you provide it, unless you state otherwise. Your contact details will never be published. Please ensure you have read the terms and conditions.
If you are reading this page on the BBC News app, you will need to visit the mobile version of the BBC website to submit your question on this topic.
MPs are likely to have the chance, and possibly multiple chances, to vote on Prime Minister Boris Johnson's Brexit deal in the next few days - but what would voters like them to do?
There are some key clues from a clutch of polls since the deal was unveiled last week.
Voters' support for Boris Johnson's deal
Three companies have asked voters whether they support or oppose what Mr Johnson brought back from Brussels.
All found many voters - between about three in 10 and four in 10 - said they did not know, or had not heard, anything about what had been agreed.
More voters support the deal than oppose it (%)
These figures are a reminder many voters have not necessarily been following the drama of the past week closely.
However, in each poll, rather more say they support Mr Johnson's deal than indicate they are opposed to it.
But given the high proportion of don't knows, the proportion in favour does not come even close to 50%.
Voters' views on whether MPs should back the deal
A clear majority is also lacking when voters are asked whether MPs should vote for or against the deal.
According to YouGov, 41% want MPs to accept the deal, while 24% say they should reject it. In the case of Survation, 33% want MPs to approve what has been agreed, while 25% say they should not.
That said, voters are not necessarily convinced the agreement would be good for the country.
According to YouGov, slightly more (23%) believe it would be a bad deal for Britain than think it would be a good one (17%). As many as 45% feel they do not know enough to give a view either way.
Similarly, Survation reports while 25% think it would be good for Britain as a whole, 27% believe it would be bad. And just 17% believe the UK would benefit most from the deal, while 28% think the EU would.
It may be some voters feel the agreement is the best that can be achieved in the circumstances.
Either way, so far at least, it is difficult for either the government or the opposition parties to argue most voters clearly back their point of view on the deal.
Do voters prefer Boris Johnson's or Theresa May's deal?
For example, when Survation asked voters whether they supported or opposed what she had brought back from Brussels, just 16% said they backed it. Nearly twice as many (30%) were opposed.
Although the balance of opinion on this measure did become somewhat more favourable thereafter, it never tilted decisively in favour of Mrs May.
In response to exactly the same question, 31% support Mr Johnson's deal while only 25% are opposed.
Meanwhile, when YouGov first asked how MPs should vote on Theresa May's deal, just 27% said they should approve it. At that time, 42% indicated they should reject it, figures that never changed dramatically.
The equivalent figures for Mr Johnson's deal are almost the mirror image of these.
It was always difficult for Mrs May to argue with MPs that voters were keen for them to back her deal.
On this score, at least, Mr Johnson appears to have a better case.
Support for Boris Johnson's deal among Leave and Remain supporters
Leave and Remain voters have very different views.
Both those who voted Leave and those who backed Remain in 2016 were unhappy with what Mrs May had brought back from Brussels.
Now, it looks as though at least half of those who voted Leave in 2016 - and maybe more - support the prime minister's deal.
But, conversely, about half of those who voted Remain are opposed to the deal.
Voters have also been asked what they would do if a referendum offered a choice between leaving the EU on the terms proposed by Mr Johnson or remaining in the EU.
Their responses reinforce the impression Mr Johnson's deal has divided the country.
Both Opinium and Survation have suggested the outcome of such a ballot would be a tie.
Many Remain supporters appear to believe if Mr Johnson's deal were to be put to a confirmatory vote, a majority would vote to stay in the EU.
However, it seems such an outcome is by no means guaranteed.
But, equally, the deal does not appear to provide a promising foundation for bringing the country together, as the prime minister argues it would.
So far, at least, he has won over too few Remain voters for that to be the case.
The delay increases the likelihood of parliamentary maneuvers that could derail or alter Johnson’s plans this week, but Johnson isn’t heading toward certain defeat. He could still secure approval for his deal, perhaps without having to make use of the Brexit extension beyond Oct. 31, which Johnson was forced to request Saturday. After more than three years of uncertainty, there might be a tight majority for his agreement.
In case his deal is approved, Johnson is likely to celebrate himself as the man who got “Brexit done” — a phrase he has frequently used in the past. But in reality, the United Kingdom’s Brexit woes are here to stay. Many of the Brexit concerns that have been on Britons’ minds in recent years — a breakup of the United Kingdom or the possibility of severe economic ramifications, for instance — would remain a risk, even if Johnson’s deal is approved.
Northern Ireland is expected to remain at the center of the Brexit debate one way or another. So far, concerns have mostly focused on a “no deal” outcome, which would trigger the introduction of border controls and could disrupt the Good Friday Agreement. That agreement ushered in a more peaceful era for Northern Ireland in the late 1990s, after a bloody conflict between Nationalists, who favored unifying with the Republic of Ireland, and Unionists, who backed remaining part of the United Kingdom.
Johnson’s proposed deal would avoid the reintroduction of border checks between Northern Ireland and the Republic of Ireland, but only because it would, in practice, move the customs border into the Irish Sea, which separates the islands of Great Britain and Ireland. Even though Northern Ireland would leave the E.U. customs union with the rest of the United Kingdom, Northern Ireland would — at least initially — in some aspects remain aligned with the European Union.
That’s a major headache for Northern Ireland’s Democratic Unionist Party (DUP), which has so far backed the Conservative Party in Parliament but won’t support Johnson’s deal. The party fears that his plans would drive Great Britain and Northern Ireland apart and could lead to Northern Ireland eventually unifying with the Republic of Ireland.
“Paradoxically Mr. Johnson and Brexit may have done more for a united Ireland than the [Irish Republican Army] ever did,” Jonathan Powell, a former adviser to prime minister and Labour party leader Tony Blair, wrote in the Financial Times over the weekend. Poll numbers indicate that support for unification is on the rise in Northern Ireland.
Those advocating a break with the United Kingdom could see a similar rise in support in Scotland, where First Minister and Scottish National Party leader Nicola Sturgeon has been rallying support for a second independence referendum. Five years ago, Scotland voted to remain part of the United Kingdom, but — like Northern Irish voters — Scots predominantly backed remaining in the E.U. in 2016.
Last week, Sturgeon reiterated that “it is clearer than ever that the best future for Scotland is one as an equal, independent European nation."
There is no indication that the approval of a Brexit deal would hurt Johnson’s critics in Northern Ireland and Scotland. In fact, the opposite might be true. If Parliament passes his deal, the basics of the United Kingdom’s current relationship with the E.U. would remain unchanged until at least the end of next year, giving negotiators time to prepare a potential free-trade deal and settle other aspects related to future E.U.-British ties.
But pro-E.U. MPs worry that hard-line Brexiteers may be hoping to derail those trade talks, in which case Britain could crash out of the current arrangements. In some ways, the economic ramifications of that would be similar to a “no deal” scenario and could result in a plummeting GDP.
Ironically, all of this — including Johnson’s deal itself — would constitute a more radical break with the E.U. than even some Brexiteers proposed back in 2016.
Since 2016, however, the British electorate appears to have moved in the exact opposite direction. Whereas few “leave” or “remain” voters appear to have changed their minds on Brexit, polls have still started to show a consistent preference for remaining in the E.U. for at least a year.
Pollsters argue that the growing support for remaining in the E.U. is mostly due to demographic change, The Post’s Karla Adam writes. Some older voters — who tend to be more supportive of Brexit — have died. Meanwhile, younger Britons who have reached the voting age since 2016 are predominantly in favor of staying in the European Union.
As the idea of a confirmatory referendum on Johnson’s deal appears to be gaining momentum among the opposition Labour Party, there is a theoretical chance that British voters’ increasingly pro-remain attitude could still derail Johnson’s plans, even if Parliament passes the deal. The emphasis here, of course, should be on “theoretical.”
Brexit has consistently proved to be unpredictable. After more than three years, there does appear to be one certainty, however: Whatever happens, Brexit won’t just be “done,” as Johnson is suggesting.
