LONDON (AP) — 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.

The truck’s driver — a 25-year-old man from Northern Ireland — was arrested on suspicion of murder. He has not been charged and his name has not been released.

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 U.K. is negotiating its divorce from the EU.

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.

Ambulance workers discovered the bodies after being called at 1:40 a.m. to a truck on the grounds of the Waterglade Industrial Park in Grays, 25 miles (40 kilometers) 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 organized 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.

Veselin Toshkov in Sofia, Bulgaria, and Pablo Gorondi in Budapest, Hungary contributed to this story.