The 39 Vietnamese migrants found dead in a refrigerated lorry near the Dartford Crossing died from a lack of oxygen and overheating, provisional investigations show.

The bodies, which included a 15-year-old boy, were found in a trailer on an industrial estate in Grays, Essex and were initially thought to be Chinese citizens.

It is thought two more lorries containing migrants were transported in to the UK via the same port just weeks before the atrocity.

Post mortem examinations have been completed and the provisional cause of death for those in the lorry was a combination of hypoxia- oxygen deprivation- and hyperthermia- overheating- in an enclosed space.

Investigators 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 migration and he is currently in custody in Essex.

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

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

He was the subject of a European Arrest Warrant and appeared at Chelmsford Magistrates' Court on Saturday.

His next appearance will be at the Old Bailey on March 16 where 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 in connection with the deaths last Tuesday.

Eamonn Harrison, 22, of Mayobridge, Northern Ireland, awaits extradition to the UK.

He also faces 39 charges of manslaughter and charges for conspiracy to traffic people and conspiracy to assist unlawful immigration and will appear in court again tomorrow.

Driver Maurice 'Mo' Robinson, 25, of Craigavon, Northern Ireland was previously 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 also been several other arrests, including in Vietnam.