Detectives investigating the deaths of 39 Vietnamese nationals whose bodies were found in a refrigerated lorry in southeast England on Friday said they had made another arrest.
Essex Police said a 23-year-old man from Northern Ireland had been held on suspicion of conspiracy to traffic people and conspiracy to assist unlawful immigration.
He was detained in the early hours of Friday on the M40 motorway, the force said in a statement.
The bodies of eight women and 31 men were found in a trailer on an industrial estate east of London, on October 23.
The lorry had earlier arrived on a cargo ferry from the Belgian port of Zeebrugge.
Lorry driver Mo Robinson, 25, has appeared in court charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration and money laundering.
He is scheduled to appear at an administrative hearing at a London court next week.
Another man from Northern Ireland, Eamonn Harrison, 22, is facing extradition proceedings for the same charges and appeared in court in Dublin on Thursday.
The case was adjourned until December 12.
Three other people have been arrested, questioned and released on bail as part of the investigation.
Essex Police detectives last month said they wanted to speak to two brothers also from Northern Ireland about the case and urged them to hand themselves in.
Many of the victims were from impoverished central Vietnam, where the main source of income is from fishing, farming or factory work.
Many families took on thousands of dollars of debt to send their children to Britain, in the hope they would land good jobs and send money back to pay off the loans.
The incident has laid bare the dangers of illegal migration in Britain, where Vietnamese nationals have often found work in nail bars or illegal cannabis farms.