The 39 Vietnamese migrants found dead in the back of a refrigerated lorry died of lack of oxygen and overheating in an enclosed space, police have said.
Among those in the container on 23 October in Grays, Essex, were 31 men and eight women. The youngest was 15.
Post-mortem examinations listed the provisional causes of death as a combination of hypoxia and hyperthermia, Essex Police said.
Lorry driver Maurice Robinson has been charged with 39 counts of manslaughter.
Seven arrests have been made in connection with the deaths across Europe and two more people have been arrested, Essex Police said.
A 22-year-old man was arrested in Northern Ireland on Sunday on suspicion of manslaughter and facilitating unlawful immigration. He is in custody in Essex.
Gheorghe Nica, 43, of Mimosa Close in Langdon Hills, Essex, was detained at Frankfurt Airport on 29 January, Essex Police said.
He was subject of a European Arrest Warrant and appeared at Chelmsford Magistrates’ Court on Saturday charged with 39 counts of manslaughter and conspiracy to assist unlawful immigration.
He will next appear at the Old Bailey on 16 March.
Eamonn Harrison, 23, of Mayobridge, Northern Ireland, appeared at Dublin’s High Court on 4 February and awaits extradition to the UK. He faces 39 charges of manslaughter as well as charges for conspiracy to traffic people and conspiracy to assist unlawful immigration. He is due to appear at the same court on Wednesday.
Robinson, of Laurel Drive, Craigavon in County Armagh, Northern Ireland, admitted conspiring to assist illegal immigration and acquiring criminal property at the Old Bailey in November. He has not yet entered pleas to the manslaughter counts or conspiracy to commit human trafficking and transferring criminal property.
His co-accused, Christopher Kennedy, 23, from Darkley, County Armagh, denied conspiracy to commit human trafficking offences, assisting unlawful immigration and arranging to facilitate the travel of other people with a view to exploitation.
Detectives want to hear from anyone who has knowledge of two similar journeys that haulage vehicles made into Purfleet on 11 and 18 October 2019. Police believe lorries were used to facilitate the unlawful entry of people into the country via the port.