GRAYS, England — Who were they, where were they going, and where were they from. It was all a mystery.
Their bodies were found after someone — officials have not said who — called an ambulance and the police came to investigate.
The passengers, all of whom save one teenager appeared to be adults, were declared dead at the scene. The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.
The authorities said they had more questions than answers, and warned of a long, complex investigation.
This was far from the first time a truckload of bodies was found in Europe, and it appeared to have all the earmarks of human trafficking. There has been a trail of victims across Europe in recent years, people desperate to make new lives but losing them instead as they were smuggled in trucks, which are often tightly sealed.
“This is a tragic incident where a large number of people have lost their lives,” Chief Superintendent Andrew Mariner of the Essex Police said in a statement. “Our enquiries are ongoing to establish what has happened.”
It could take a long time to identify the victims, given that they may not have been carrying documentation.
But even gathering information about the truck in which they met their deaths, and the route it had followed before ending up at an industrial park east of London, proved less than straightforward.
Initially, the police said they believed the truck had entered Britain at the port of Holyhead in Wales on Saturday. But they later established that only the Bulgarian-registered tractor — the front of the truck, with the driver’s cab and engine — had taken that route.
The cargo trailer it was pulling entered Britain at Purfleet, a shipping port near Grays, shortly after 12: 30 a.m. Wednesday and was met there by the driver, they said. The police believe the container in which the bodies were found came from Zeebrugge, in Belgium.
Pippa Mills, deputy chief constable of the Essex Police, said investigators were in the “early stages of what is likely to be a lengthy investigation,” and appealed for information from the public. She described the truck as a “complex scene” that was being examined by experts.
“This is an absolute tragedy and very sad day for Essex Police and the local community,” Deputy Chief Mills said at a news conference.
The truck was found at the Waterglade Industrial Park in Grays, about 25 miles east of London. Officers were called to the site shortly before 1: 40 a.m.
Ms. Mills said the police had been summoned by the local ambulance service, but she did not say who had alerted the ambulance service.
While the circumstances that led to the deaths remained unclear, the trailer appeared to be temperature-controlled. Such containers are typically tightly sealed, said Dave Wood, a former director general of British immigration enforcement, creating a risk of suffocation for anyone traveling inside.
Human trafficking in Britain, Mr. Wood said, is “a growing industry and a major outlet for organized crime.”
But he said more and more of the trafficking into Britain lately had used small, inflatable boats to cross the English Channel.
Security checks have increased at major crossing points like Dover and Calais, so smugglers might see less risk in traveling from other European ports to the smaller port in Purfleet, which mainly handles freight.
A spokeswoman for the Bulgarian Ministry of Foreign Affairs said the truck tractor found at the industrial park was registered in Varna, a major Black Sea port, under the name of an Irish citizen. But Bulgaria’s prime minister told a local news outlet that the truck had not been in the country since it was registered there in 2017, and it was likely that the license plate was its only Bulgarian connection.
There have been a string of tragedies involving migrants and laborers being smuggled across Europe in trucks. In 2015, the decomposing bodies of 71 migrants from the Middle East and Afghanistan were discovered in a truck abandoned on the side of an Austrian highway, one of several such cases in the country.
In 2000, 58 Chinese migrants were found suffocated in a truck in Dover, in southeastern England, after crossing from mainland Europe. Just days ago, five people were arrested, including four men in Britain, after 13 migrants including a child were discovered in a cattle truck in Calais, en route to Britain.
As details of the grisly discovery in Essex were emerging, the news drew expressions of outrage and horror from many in Britain, including Prime Minister Boris Johnson, who described himself as “appalled.”
Priti Patel, the British cabinet minister responsible for policing and immigration, said she was “shocked & saddened by this utterly tragic incident.”
Ms. Patel has been a vocal proponent of stronger immigration enforcement, vowing a hard-line stance once Britain leaves the European Union, and her comments were quickly met with anger from some fellow Britons.
“We need more than empty expressions of shock and sadness from Priti Patel and Boris Johnson,” said Satbir Singh, chief executive of the Joint Council for the Welfare of Immigrants, a British charity focused on immigrants’ rights.
In a statement, Mr. Singh said, “The ultimate responsibility for these deaths lies with government policy which has deliberately closed down safe and legal routes into Britain.” “People move, they always have and they always will,” he said. “Nobody should have to risk their lives to do so.”
The industrial park is a short drive from the busy Dartford Crossing, where the main highway encircling London passes under and over the River Thames. By midday Wednesday, several police officers guarded a part of Eastern Avenue that was cordoned off with temporary fences covered with green tarpaulin.
The area is home to a few dozen warehouses, car-repair shops and freight companies, and some who work in the area were reeling at the discovery of so many dead.
“We would have never expected such tragedy to happen here, in this zone,” said Ryan Sharpe, a 24-year-old driver at Transmec Group, a nearby freight company.
Catalin Bortoc, 28, works at an auto parts company just across from where the truck was being examined.
“Take any industrial estate you’d find in the U.K. and in Europe — except you add a lorry with 39 bodies inside,” he said. “It’s just so awful.”
Britain has long been a destination for migrants, and smugglers have often used English Channel crossings to traffic people into the country.
Most covert transport methods pose significant safety risks, according to Britain’s National Crime Agency. Migrants often travel in shipping containers or commercial vehicles that are transported by truck, rail or ferry, or they cross on small boats.
Organized gangs frequently smuggle people in hard-sided trucks like the one in Essex, while small-time traffickers tend to use soft-sided trucks.
The crime agency has acknowledged that the length of the Britain’s coastline and the sheer volume of passenger and freight traffic into the country make identifying illegal migration “a significant challenge.”
Elian Peltier reported from Grays, England, and Megan Specia from London. Benjamin Mueller contributed reporting from London, Boryana Dzhambazova from Sofia, Bulgaria, and Ed O’Loughlin from Dublin.