LONDON — Authorities said Thursday that the 39 people found dead inside a tractor-trailer at an industrial park in southeastern England are Chinese nationals, probably the victims of gangs who smuggle human cargo into Britain via shipping containers from European sea ports.

It is one of Britain’s biggest ever homicide cases, with investigations extending to Belgium, Ireland, Bulgaria and China.

Experts who follow human trafficking trends suggested those who were found dead could have been compelled into forced labor. Or they could have been migrants who paid their way for the dangerous journey gone horribly wrong.

Police detained the 25-year-old driver of the refrigerated truck on suspicion of murder and on Thursday searched three properties in Northern Ireland, where he is from.

Britain’s National Crime Agency said it was working to identify “organized crime groups who may have played a part.”

China’s ambassador to Britain, Liu Xiaoming, said in a statement that the Chinese Embassy in London has sent a team of officials to Essex, England. The Chinese diplomats “have met with the local police, who said that they are verifying the identity of the 39 deceased, whose nationality still cannot be confirmed,” the ambassador said.

The bodies of the 39 people — eight women and 31 men — were found at Waterglade Industrial Park in Grays, about 25 miles east of central London. 

Essex Police and Belgian prosecutors said the refrigerated container arrived by sea from Zeebrugge, a Belgian port, and docked at Purfleet, a small port in Essex on the River Thames, shortly after 12:30 a.m. Wednesday. 

Police said the truck that picked up the container at Purfleet entered Britain separately, via ferry from Dublin, at Holyhead port in North Wales on Sunday.

About 1:05 a.m. Wednesday, the truck and trailer left Purfleet, police said. Thirty-five minutes later, police received a call from local ambulance services saying they had discovered the container. It was unclear how the ambulance services had been alerted.

The Belgian Federal Public Prosecutor’s Office announced it had opened a case to focus on who was behind the transport. The Belgian investigators said they did not know when the migrants were loaded into the container. Nor have police said how the migrants got from China to Belgium.

The case echoed a fatal episode in June 2000, when the bodies of 58 Chinese migrants were found in a shipping container in the English port city of Dover. Those victims were supposedly destined to work in Britain as domestic servants, authorities said. The following year, a Dutch driver was sentenced to 14 years in jail for manslaughter. The immigrants, who paid a smuggling gang $26,000, suffocated after the driver closed a vent on the truck during a five-hour ferry ride across the English Channel.

In February 2004, at least 21 Chinese migrants who were picking cockles on the coast in the dark were caught in treacherous tides and killed. The event, in Morecambe Bay, shone a spotlight on illegal forced laborers smuggled into Britain.

Lucy Moreton of the Immigration Services Union, which represents British border, customs and immigration officers, said that desperate attempts to reach Britain appeared to be on the rise. She described a form of modern-day slavery in which Chinese, Bangladeshi, Vietnamese and others allow themselves to be shipped to Britain, where they toiled for years in construction, domestic service, agriculture, nail salons and the sex trade to “pay back” the debt they owed gangs for smuggling them abroad. 

Richard Burnett, chief executive of the Road Haulage Association, said that there were not enough security checks in many European ports and that this could be exacerbated with Brexit.

“We’ve found it hard enough to try and get the Europeans to accept that this is a collective problem,” he said. “There are no incentives for the Europeans to really help us if we’re exiting the U.K. Their view is that this is our problem because these people want to get to the U.K.”

Police on Thursday did not name the driver, though several British media outlets have identified him, citing sources in Northern Ireland, and posted photos from what were said to be his social media accounts.

“We have not speculated about the identity of this man, and we will not do so,” Essex police said in a statement Thursday.

The truck was registered in Varna, Bulgaria — a port city on the Black Sea — to a company owned by an Irish citizen, according to a statement by the Bulgarian Foreign Ministry. Bulgarian Prime Minister Boyko Borissov told a local television broadcaster that the truck left immediately after it was registered in 2017 and hadn’t returned.

British officials had previously issued warnings about the route the container took across the North Sea.

In 2016, Britain’s Border Force identified Zeebrugge, along with the Hook of Holland, as key ports for “clandestine arrivals” into Britain. 

In a report published last year, Britain’s National Crime Agency said Belgium had “become a location of greater focus” for human trafficking and that smuggling activity increased following the closure of a migrant camp in Dunkirk, France.

Analysts said Zeebrugge had been selected in this case because it lacks many of the security measures — including sniffer dogs and high-tech monitors — employed in French ports.

Rick Noack in Berlin and Anna Fifield in Beijing contributed to this report.