Two brothers are wanted on suspicion of manslaughter after 39 people were found dead in a refrigerated trailer in Essex last week.
Ronan Hughes, 40, and Christopher Hughes, 34, both from Armagh in Northern Ireland, are also wanted on suspicion of human trafficking, Essex Police said on Tuesday.
The older brother also goes by the name Rowan, and both have links to the Republic of Ireland as well as Northern Ireland.
Police launched an investigation after the bodies of eight women and 31 men were found in the refrigerated trailer at the Waterglade Retail Park in Grays, Essex, in the early hours of Wednesday.
Detective Chief Superintendent Stuart Hooper, who is leading the investigation, said: "Finding and speaking to the Hughes brothers is crucial to our investigation.
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"At this time we believe they are in Northern Ireland but they also have links to the Irish Republic.
"If you know where they are or have any information about their whereabouts I need you to call my team."
More from Essex Lorry Deaths
Belgian officials said the trailer where the victims were found came from Zeebrugge on Tuesday and was picked up by a lorry cab in Purfleet, Essex at about 12.30am on Wednesday.
Essex Police are now investigating whether a lorry cab seized at Dublin Port by Irish police was the lorry that delivered the refrigerated trailer to Zeebrugge port last Tuesday with the 39 people on board.
They confirmed a man was arrested by Irish police over an unconnected matter and he "is a person of interest" in their investigation into the deaths.
The force said it was liaising with Irish officers as the man is outside England and Wales' jurisdiction.
On Monday lorry driver Maurice "Mo" Robinson, 25, of Craigavon, Northern Ireland, appeared at Chelmsford Magistrates' Court charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration, and money laundering.
He is due to appear at the Old Bailey on 25 November.
Three other people have been arrested – a 38-year-old man and a 38-year-old woman from Warrington, Cheshire, and a 46-year-old Northern Irish man who was arrested at Stansted Airport on Friday.
They have all been released on bail until next month.
Detectives are now trying to identify the people who died in the trailer, in the largest identification process in Essex Police history.
The lorry was moved to Tilbury Docks after the initial discovery and the bodies have now been moved to Broomfield Hospital in Chelmsford, for post-mortems to be carried out.
