Eight people have been arrested in Vietnam in connection with the 39 migrants found dead in a lorry in Essex.
The group, arrested by Vietnamese police, were held on Sunday on suspicion of organising people smuggling overseas.
It is believed the 31 men and eight women whose bodies were discovered in the back of the refrigerated truck in the UK two weeks ago were Vietnamese, despite initial reports from police that they were Chinese.
A spokeswoman for Vietnam's Ministry of Foreign Affairs called the incident a "serious humanitarian tragedy".
Two people have been charged to date over the case.
The driver of the lorry, 25-year-old Mo Robinson, appeared at Chelmsford Magistrates' Court charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration and money laundering.
And Essex Police have started extradition proceedings to bring 22-year-old Eamonn Harrison from Ireland to the UK.
He is charged with 39 counts of manslaughter, along with human trafficking and immigration offences at Dublin High Court, and was remanded in custody.
Two men from Armagh in Northern Ireland are also wanted on suspicion of manslaughter and human trafficking.
The pair, 40-year-old Ronan Hughes and his brother Christopher, 34, are believed to have links with the road haulage and shipping industries, and detectives have urged them to hand themselves in.