The 39 people found dead inside a refrigerated truck in southeastern England are believed to be Chinese nationals, British police said on Thursday.

Emergency workers found the bodies of 39 people inside a truck container at an industrial estate in Grays in Essex on Wednesday morning.

Police confirmed the victims included 31 men and eight women. A victim previously thought to be a teenager was actually a young adult woman.

The alleged driver, a 25-year-old man from Northern Ireland, was arrested on suspicion of murder after driving the truck to Grays, about 32 kilometers (20 miles) east London. He has not been charged.

Police have raided three properties in Northern Ireland as part of the investigation.

The circumstances leading up to their deaths have not been revealed, but illegal immigrants are often hidden on trucks in order to reach the United Kingdom from the European mainland.

Echoes of previous tragedies

In the country's biggest illegal immigrant tragedy, 58 Chinese people were found dead in a tomato truck at the southern port of Dover in 2000. The vehicle had begun its journey to the UK in Belgium's Zeebrugge port, as appears to be the case in the latest incident.

Police said the container section of the vehicle came by ferry from the Belgian port into Purfleet, close to where the truck was found. The ferry crossing takes nine to 12 hours. The tractor unit of the vehicle, registered in Bulgaria, was believed to have originated in Northern Ireland.

Forensic investigators were working by the truck throughout Wednesday, and it was later moved to a nearby "secure location", so "the bodies can be recovered whilst preserving the dignity of the victims," police said. It is the country's largest murder investigation since the 2005 terrorist attacks, which killed 52 people.

Prime Minister Boris Johnson described it as "unimaginable tragedy."

Underscoring the issue of people being smuggled in trucks, police in Kent discovered nine people, alive, stowed in the back of a truck on Wednesday.

aw/rt (AP, Reuters, dpa, AFP)