British police say the 39 bodies that were discovered in a refrigerated trailer in Essex are all believed to be Chinese nationals. An investigation into possible human smuggling and murder was sparked when the truck container was found east of London on Wednesday.

The truck's driver, a 25-year-old man from Northern Ireland, remains in custody after being arrested on suspicion of murder. On Thursday, police secured a warrant to hold him for up to an additional 24 hours as they continue to investigate.

British media are reporting the driver's name is Mo Robinson; police have refused to confirm publicly whether that is the driver's identity.

"We continue to ask journalists and those on social media not to speculate about the identity of the lorry driver," Deputy Chief Constable Pippa Mills said, adding, "This is an incredibly sensitive and high-profile investigation, and we are working swiftly to gather as full a picture as possible as to how these people lost their lives."

Giving new details about the case on Thursday, police say they believe 38 of the people were adults and that one was a young adult woman — who was initially reported to have been a teenager. Eight of the people were women, and 31 were men.

"We read with heavy heart the reports about the death of 39 people in Essex, England," the Chinese Embassy in the U.K. said in a statement. It added that Chinese officials are in "close contact with the British police" to clarify and confirm what happened.

"Authorities will conduct autopsies to determine the exact cause of death and then try to identify the victims, which could take weeks," NPR's Frank Langfitt reports from London. He adds, "Britain's national crime agency is trying to identify the criminal gangs who may have been behind the smuggling operation."

In another update to the case, investigators now say the truck and trailer did not enter the U.K. together.

The truck traveled from Dublin to the Welsh ferry port of Holyhead on Sunday, police say. Meanwhile, the trailer traveled from the Belgian port of Zeebrugge to the Essex port of Purfleet on the Thames River, arriving just after midnight local time Wednesday morning. The two were coupled together and left the port less than an hour later, police say.

Police were called to the scene at the Waterglade Industrial Park by medical personnel at 1:40 a.m. (8:40 p.m. ET) Wednesday, when they made the tragic discovery of dozens of bodies in the container.

On Thursday, police confirmed that three properties in County Armagh, Ireland, have been searched in connection with their investigation.

The deaths of dozens of people raised echoes of another harrowing incident from 2000, when the bodies of 58 Chinese migrants were found suffocated in a trailer at the English port of Dover. As in that case, the container had traveled to the U.K. from Zeebrugge.

In the 2000 case, the driver of the Dutch-registered truck was eventually sentenced to 14 years in prison. Several years later, a Dutch court sentenced seven members of a Chinese gang to up to six years in prison on charges related to the ensuing inquiry.

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