British police said on Tuesday that two more arrests had been made over the deaths of 39 Vietnamese migrants found in the back of a truck near London last year while investigations indicated they had died of overheating and lack of oxygen.
The victims, who included two 15-year-old boys, were found on an industrial estate in Grays in Essex, about 20 miles (32 km) east of London in October.
Mostly from Nghe An and Ha Tinh provinces in north-central Vietnam, their deaths shone a light on the human smuggling trade.
Autopsies had concluded that the provisional cause of death was a combination of hypoxia - oxygen deprivation - and hyperthermia - overheating - in an enclosed space.
Essex Police said a 22-year-old man had been arrested in Northern Ireland on Sunday on suspicion of manslaughter and facilitating unlawful immigration. He was now in custody in Essex.
Last week, British police along with German authorities detained Gheorghe Nica, 43, who was wanted on a European Arrest Warrant, at Frankfurt Airport.
Nica, who lives near Grays, appeared in court on Saturday accused of 39 counts of manslaughter and one count of conspiracy to assist unlawful immigration and is due to reappear at London's Old Bailey court on March 16.
"Our teams are continuing to progress hundreds of lines of enquiry and are working with the National Crime Agency and other law enforcement agencies from across the globe to further their lengthy and complex investigation," Essex Police said.
Maurice Robinson, the British driver of the truck who hailed from Northern Ireland, admitted last November plotting to assist unlawful immigration and acquiring criminal property.
British authorities are also trying to extradite Eamonn Harrison, 23, from Ireland on charges of manslaughter, human trafficking and immigration offences. He is due at Dublin's High Court on Wednesday.
Police have said the Vietnamese victims were found not long after the container arrived in Britain from Zeebrugge in Belgium. The refrigerated unit was picked up at Purfleet dock, not far from Grays, while police believe the truck cab was driven over from Ireland.