The 39 Vietnamese migrants found dead in a lorry next to the Dartford Crossing on an Essex industrial estate died as a result of overheating, provisional investigations show.

The bodies, which included a 15-year-old boy, were found in a trailer in Grays in October and were initially thought to be those of Chinese citizens.

It is also thought two more lorry loads of migrants were transported in to the UK via the same port in the weeks before.

All post-mortem examinations have now been completed and while Essex Police await the final reports the provisional cause of death for those in the lorry was a combination of hypoxia and hyperthermia (overheating) in an enclosed space.

Investigators have also made further arrests in connection with the manslaughter investigation.

A 22-year-old man was arrested in Northern Ireland on Sunday on suspicion of manslaughter and facilitating unlawful immigration. He is currently in custody in Essex.

Last week, on the instruction of Essex Police, German authorities and the National Crime Agency carried out an arrest.

Gheorghe Nica, 43, of Mimosa Close in Langdon Hills, Basildon, was detained at Frankfurt Airport on Wednesday, January 29.

Nica was subject of a European Arrest Warrant and appeared at Chelmsford Magistrates’ Court on Saturday. He will next appear at the Old Bailey on March 16.

He faces charges of 39 counts of manslaughter and one count of conspiracy to assist unlawful immigration.

A second man appeared in Dublin’s High Court last Tuesday in connection with the deaths.

Eamonn Harrison, 23, of Mayobridge, Northern Ireland awaits extradition to the UK under the terms of a European Arrest Warrant.

Harrison also faces 39 charges of manslaughter and charges for conspiracy to traffic people and conspiracy to assist unlawful immigration.

He will appear at the same court again tomorrow.

Driver Maurice 'Mo' Robinson, 25, of Craigavon, Northern Ireland, was previously charged with 39 counts of manslaughter and has admitted assisting unlawful immigration and acquiring criminal property.

Christopher Kennedy, 23, of County Armagh, has pleaded not guilty to conspiracy to commit human trafficking and assisting unlawful immigration, and is due to go on trial next year.

There have been several other arrests including in Vietnam.

Police are keen to hear from anyone who has knowledge of two similar journeys that haulage vehicles made into Purfleet on Friday October 11 and Friday October 18. It is believed that lorries were used to facilitate the unlawful entry of people into the country via Purfleet.

You can call Rayleigh’s Major Crime Team on 101.

To get the latest updates on ongoing cases, police appeals and criminals put behind bars, click here.