LONDON, United Kingdom (AP) — All 39 people found dead in a container truck near an English port were Chinese citizens, British police confirmed Thursday as they pressed to investigate 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 the Waterglade Industrial Park in Grays, a town 25 miles (40 kilometres) east of London. The force says one victim previously thought to be a teenager was instead a young adult woman.

The 25-year-old truck driver, who is from Northern Ireland, is being questioned on suspicion of attempted murder but has not been charged. Police in Northern Ireland have searched three properties as they sought to piece together the journey of the truck and the victims.

The Chinese Foreign Ministry posted a statement on its social media account saying that Chinese embassy employees in the UK were driving to the scene of the crime to aid the investigation.

Britain remains an attractive destination for immigrants, even as the UK is negotiating its divorce from the European Union. Upon hearing of the deaths, British Prime Minister Boris Johnson vowed in Parliament on Wednesday that people smugglers would be prosecuted to the full extent of the law.

The tragedy recalls the deaths of 58 people in 2000 in a truck in Dover, England, who had undertaken a perilous, months-long journey from China’s southern Fujian province and were discovered stowed away with a cargo of tomatoes after a ferry ride from Zeebrugge, the same Belgian port used in the latest tragedy.

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

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 truck and the trailer with the people inside apparently took separate journeys before ending up at the industrial park. British police said they believe the container went from the port of Zeebrugge to Purfleet, England, where it arrived early Wednesday.

UK authorities have warned for several years that people smugglers are turning to Dutch and Belgian ports because of increased security on the busiest trade route between 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 organised immigration crime threat.” The same year, the UK Border Force identified Zeebrugge and the Hook of Holland in the Netherlands as key launching points for people trafficking into Britain.