Investigators were trying to piece together the movements of a large cargo truck found Wednesday containing the bodies of 39 people in one of Britain's worst people smuggling tragedies.
Details about the victims, including where they were from, were scarce. Police in southeast England said they have not been identified — a process they warned would be slow.
Police were called to the scene and Mo Robinson, a 25-year-old man from Northern Ireland, who was the driver of the vehicle, was arrested on suspicion of murder.
He and other drivers who may have been at the wheel before him would have taken advantage of the European Union's generally open borders to travel in several countries without border checks. Britain remains an attractive destination for immigrants, even as the UK is negotiating its divorce from the EU.
Road Haulage Association chief executive Richard Burnett told the Irish Mirror the style of container appeared to be a refrigerated unit which would have been "absolutely horrendous" for anyone trapped inside.
Temperatures can reach as low as -25 degrees for frozen products, causing humans to "lose their lives pretty quickly," he said.
"It's going to be dark. If the fridge is running it's going to be incredibly cold.
"The only place to go to the toilet is on board the back of the trailer. You can imagine if they've been in there for days then there will be faeces, there will be urine," he said.
Deputy chief constable Pippa Mills from Essex Police said it was "an absolute tragedy and a very sad day" for officers and the community.
"We believe the lorry is from Bulgaria and came in to the UK through Holyhead on the 19th of October," she told reporters at a press conference.
"At this stage we have not identified where the victims are from or their identities, and we anticipate that this could be a lengthy process."
In Parliament, Prime Minister Boris Johnson put aside the Brexit crisis, at least for a few minutes, and vowed that the people traffickers would be found and prosecuted to the full extent of the law.
"All such traders in human beings should be hunted down and brought to justice," he said.
Jackie Doyle-Price, a member of Parliament who represents the area where the truck was found, said: "To put 39 people into a locked metal container shows a contempt for human life that is evil. The best thing we can do in memory of those victims is to find the perpetrators and bring them to justice."
Ambulance workers discovered the bodies after being called at 1.40am to a truck on the grounds of the Waterglade Industrial Park in Grays, 40 kilometres east of London on the River Thames. It was unclear who called the ambulance service.
No cause of death has been made public. Police said one victim appeared to be a teenager but gave no further details.
Police initially said the cargo truck had traveled through Ireland and then to Wales via ferry. But Essex police later said they believe the container with the people inside went from the port of Zeebrugge in Belgium to Purfleet, England, where it arrived early Wednesday. Police said they believe the tractor unit traveled from Northern Ireland and picked up the container unit.
"This is a tragic incident where a large number of people have lost their lives. Our inquiries are ongoing to establish what has happened," Essex Police Chief Superintendent Andrew Mariner told reporters.
The cargo truck had a Bulgaria registration, Bulgaria's Foreign Ministry said in a news release. But Bulgarian authorities said they could not yet confirm that the truck had started its journey there. The Foreign Ministry said the Swedish-made "Scania" truck was registered in the Bulgarian Black Sea port city of Varna to a company owned by a woman from Ireland.
"We are in contact with our embassy in London and with British authorities," Foreign Ministry spokeswoman Tsvetana Krasteva said.
The number of victims was shocking, although it has become sadly common in recent years for small numbers of migrants to occasionally be found dead in sealed vehicles after having been abandoned by traffickers.
The tragedy recalls the death of 58 migrants in 2000 in a truck in Dover, England, and the deaths in 2015 of 71 migrants from Syria, Iraq and Afghanistan who were found suffocated in the back of a refrigerated truck that was abandoned on an Austrian highway close to the Hungarian border.
It seems likely the traffickers shunned the most popular English Channel route from Calais, France, to Dover, England because of increased surveillance at those ports and instead chose a more circuitous route.
Dover and Calais, which have been under pressure from human traffickers for years, have sniffer dogs, monitors and more advanced technological surveillance due to the fact that they are the endpoints for the Channel Tunnel between France and Britain.
Groups of migrants have repeatedly landed on English shores using small boats for the risky Channel crossing, and migrants are sometimes found in the trunks of cars that disembark from the massive ferries that link France and England, but Wednesday's macabre find in an industrial park was a reminder that trafficking gangs are still profiting from the human trade.
"To put 39 people into a locked metal container shows a contempt for human life that is evil," lawmaker Jackie Doyle-Price, who represents the region in parliament, told Parliament.
The National Crime Agency said its specialists were working to "urgently identify and take action against any organised crime groups who have played a role in causing these deaths."
It said in May that the number of people being smuggled into Britain via cargo trucks was on the rise.