LONDON (AP) — All 39 people found dead in a refrigerated container truck near an English port were Chinese citizens, British police confirmed today as they investigated one of the country’s deadliest cases of people smuggling.

The Essex Police force said 31 men and eight women were found dead in the truck early Wednesday at an industrial park in Grays, a town 25 miles  east of London.

The 25-year-old truck driver, who is from Northern Ireland, was being questioned on suspicion of attempted murder but has not been charged. Police in Northern Ireland also searched three properties as they sought to piece together how the truck’s cab, its container and the victims came together on such a deadly journey.

The truck and container apparently took separate journeys before ending up at the industrial park. British police said they believe the container went from the Belgian port of Zeebrugge to Purfleet, England, where it arrived early Wednesday and was picked up by the truck driver and driven the few miles to Grays.

The truck cab, which is registered in Bulgaria to a company owned by an Irish woman, is believed to have come from Northern Ireland, then headed to Dublin to catch a ferry to Wales before driving across Britain to pick up the container.

The Chinese Foreign Ministry said Chinese embassy employees in the U.K. were driving to the scene of the crime to aid the investigation and Belgian police were trying to track down information from Zeebrugge.

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 criminal gangs are still profiting from large-scale trafficking.

The tragedy recalls the deaths of 58 Chinese migrants who suffocated in a truck in Dover, England after a perilous, months-long journey from China’s southern Fujian province. They were found stowed away with a cargo of tomatoes after a ferry ride from Zeebrugge, the same Belgian port that featured in the latest tragedy.

British Prime Minister Boris Johnson vowed in Parliament that people smugglers would be prosecuted to the full extent of the law. English-speaking Britain, with its high demand for tourism, restaurant and agricultural workers, remains a very attractive destination for immigrants from all countries, even as the U.K. is rethinking its immigration rules as it prepares to leave the 28-nation European Union.

Nando Sigona, a professor of migration studies at the University of Birmingham, said tougher migration controls born of populist anti-immigrant sentiment across Europe are closing down less dangerous routes to the West and encouraging smugglers to take more risks and try out new routes.

“The fact that all these people came from the same country could hint to a more organized crime scenario,” he told The Associated Press. “Usually, if it’s an ad hoc arrangement at the port, you would get a bit of a mix of nationalities.”

Smugglers — many of whom are paid their final installment only when the person is delivered to his destination — earn more by packing as many people as possible into a ship or truck.

“Death is a side effect,” he said.

In February 2004, 21 Chinese migrants — also from Fujian — who were working as cockle-pickers in Britain drowned when they were caught by treacherous tides in Morecambe Bay in northwest England.

Sigona, who has studied Chinese immigrants to the U.K., said China’s rising middle class has more access to multiple routes to come to the West legally — say, with student or tourist visas. This means that the West is now closer to the public imagination in China, and could prompt those with fewer resources might put themselves and their families into debt in hopes of reaping similar rewards.

Belgian authorities said they had not made much headway in finding out how the container ended up in Zeebrugge.

“Up till now, we have a lot of questions and not a lot of answers. We don’t even know which road was followed by the truck in Belgium,” said Eric Van Duyse, spokesman for the Belgian prosecutor’s office. “We don’t know how much time it stayed in Belgian territory. We don’t know if it stopped or not.”

U.K. authorities have warned for several years that people smugglers are turning to Dutch and Belgian ports because of increased security measures on the busiest cross-Channel trade route between the ports of Calais in France and Dover in England.

Britain’s National Crime Agency warned in 2016 that people smuggling using containers on ferries was “the highest-priority organized immigration crime threat.” The same year, the U.K. Border Force identified Zeebrugge and the Hook of Holland in the Netherlands as key launching points for smuggling people into Britain.