39 people found dead inside the back of a haulage truck on Wednesday are Chinese nationals.

Essex Police found the bodies at 1:40 a.m local time in a truck parked at an industrial complex in Essex, 25 miles east of London.

The driver, named in several outlets as Mo Robinson, 25, was arrested on suspicion of murder.

Authorities have not yet said whether the deceased — 38 adults and one teen —were being trafficked.

The UK's National Crime Agency said it is investigating any "organized crime groups who may have played a part."

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All 39 people found dead inside the back of a truck in England on Wednesday are said to be Chinese nationals, Essex Police announced on Thursday.

Police discovered the bodies at 1:40 a.m local time after they were called by ambulance to the Waterglade Industrial Park in Grays, Essex, 25 miles east of London. Police said all 39 people were dead when they were discovered.

The truck's driver, identified by the Guardian and BBC as Northern Irish citizen Mo Robinson, 25, was arrested on suspicion of murder.

The truck at the Waterglade Industrial Estate. Sky News

So far authorities have not said whether the victims, eight women and 31 men, were being trafficked.

The UK's National Crime Agency has provided officers to identify any "organized crime groups who may have played a part," the BBC said.

The front part of the truck (called the tractor) was registered in Bulgaria and had entered the UK at Holyhead port, north Wales, on Tuesday.

But the tanker it carried arrived from Zeebrugge, Belgium, on Wednesday in Purfleet, three miles from where the bodies were found, where it was collected by Robinson.

Bulgaria's foreign ministry said the Scania truck was "registered in the city of Varna under the name of a company owned by an Irish citizen," Reuters reported.

The location of Grays, Essex. Google Maps

Belgian authorities said on Thursday that they had opened an inquiry into the deaths, the Guardian reported.

Essex Police chief superintendent Andrew Mariner said: "This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened."

A spokesperson for the Chinese embassy in the UK said on Thursday: "We read with heavy heart the reports about the death of 39 people in Essex, England."

In 2000, 58 Chinese citizens were found dead in a truck container in Dover, Kent.