Two brothers are wanted on suspicion of manslaughter over 39 people found dead in a lorry in Essex.
Ronan Hughes, 40, and Christopher Hughes, 34, from Armagh in Northern Ireland, are also wanted by police on suspicion of human trafficking.
The 39 bodies were found in the lorry trailer on an industrial estate in Grays on Wednesday.
Police believe the Hughes brothers are in Northern Ireland but say they have links to the Republic of Ireland.
The driver of the lorry, Maurice Robinson, also from Northern Ireland, appeared in court on Monday charged with a string of offences, including 39 counts of manslaughter.
Det Ch Supt Stuart Hooper said: "Finding and speaking to the Hughes brothers is crucial to our investigation.
"This is a case where 39 men and women have tragically died and support from the community is going to be vital to help bring those responsible to justice."
It is understood that the Hughes brothers are originally from Co Monaghan but have at least one business address in Northern Ireland.
Essex Police said Ronan Hughes also goes by the name Rowan.
The force asked for anyone with information about the brothers' whereabouts to contact officers.
Three other people arrested - a 38-year-old man, a 38-year-old woman, and a 46-year-old man - have all been released on bail.
Another driver, who is suspected of taking the container to Zeebrugge before it crossed the Channel, was arrested at the port of Dublin on Saturday on unrelated charges.
Irish police, who are liaising with their Essex counterparts, said the man was the driver of a blue truck seized as part of the ongoing investigations into the 39 deaths.
Police initially believed the 31 men and eight women discovered dead in the refrigerated container in the early hours of Wednesday morning were Chinese.
But a number of Vietnamese families have come forward fearing their loved ones are among the dead.
Pham Thi Tra My, 26, sent her family a message on Tuesday night, saying her "trip to a foreign land has failed".
The BBC understands from Vietnamese government sources that officers from national and provincial police departments are ready to travel to the UK to assist with the inquiry.
Vietnamese police have taken DNA samples from the hair and fingernails of some of the relatives of suspected victims to help with the identification process.
News agency Reuters said British police had sent documents relating to four of the dead to Vietnamese police.
It reported that Vietnam's deputy foreign minister Nguyen Quoc Cuong said his country and Britain were "trying to speed up identification of the bodies, but no deadline has been set".
The bodies have all been moved to Broomfield Hospital in Chelmsford where they are being examined to establish cause of death.
A spokesman for the Essex coroner's office told the BBC: "At this stage work is still ongoing to identify the 39 victims and it is not possible to put a timescale on this.
"The coroner will consider matters relating to inquests in due course."