They were held on Sunday on suspicion of organising people smuggling overseas.
The 31 men and eight women who were discovered in the back of the refrigerated truck in Grays in October are thought to have been Vietnamese.
So far two people have been charged.
Essex Police have started extradition proceedings to bring 22-year-old Eamonn Harrison from Ireland to the UK.
Harrison, of Newry in Co Down, Northern Ireland, appeared at Dublin High Court last Friday charged with 39 counts of manslaughter, along with human trafficking and immigration offences, and was remanded in custody.
The driver of the lorry, Mo Robinson, 25, appeared at Chelmsford Magistrates’ Court charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration and money laundering.
Detectives have also urged Ronan Hughes, 40, and his brother Christopher, 34,said to have links with the road haulage and shipping industries, to hand themselves in.
The pair, from Armagh in Northern Ireland, are wanted on suspicion of manslaughter and human trafficking.