Thirty-nine Vietnamese people found dead inside a lorry in Essex are believed to have suffocated and overheated, police have revealed, as further arrests were made.

Postmortem examinations have been completed and the provisional cause of death has been given as a combination of hypoxia and hyperthermia in an enclosed space, Essex police said.

It had been speculated that the victims froze to death as they were found in a refrigerated trailer. Final reports will be filed in due course.

Ten teenagers were among those found dead in the lorry on 23 October in an industrial park in Grays, Essex, having entered the country via Purfleet docks from Zeebrugge in Belgium.

The lorry driver, Maurice Robinson, was charged with 39 counts of manslaughter.

Further arrests have been made in connection with the investigation. Gheorghe Nica, 43, of Basildon, was detained at Frankfurt airport on 29 January, and was charged with 39 counts of manslaughter and one count of conspiracy to assist unlawful immigration.

Nica was the subject of a European arrest warrant. He appeared at Chelmsford magistrates court on 8 February and will appear at the Old Bailey, London, on 16 March.

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

Others have been charged in connection with the deaths. Christopher Kennedy, 23, of Darkley, County Armagh, was charged with offences of conspiracy to arrange or facilitate the travel of people with a view to exploitation and conspiracy to facilitate the commission of a breach of UK immigration law.

Eamonn Harrison, 23, of Mayobridge, Northern Ireland, faces charges of 39 counts of manslaughter, as well as human trafficking and immigration offences, but awaits extradition from Ireland to the UK. He will appear in court in Dublin on Wednesday.