All 39 migrants found dead in the back of a refrigerated lorry trailer in Essex were Chinese nationals, say sources.
The bodies of 38 adults and one teenager were discovered inside the container at an industrial estate in Grays in the early hours of Wednesday morning shortly after it arrived on a ferry from Belgium.
Police have arrested a 25-year-old driver - named locally as Mo Robinson of Portadown, Northern Ireland - on suspicion of murder and raided two properties in Co Armagh.
Officers have started the grim job of removing the bodies from the lorry and the painstaking task of trying to identify them.
It is feared the victims froze to death inside the trailer where temperatures are said to get as low as -25C.
It is believed the migrants had been inside for more than 12 hours by the time they were found dead.
All 39 were Chinese nationals, sources told PA.
Police probing one of the UK's biggest murder inquiries continue to question Robinson - whose girlfriend is said to be expecting twins - on Thursday.
Sources told the Telegraph it is "very unlikely" Robinson knew about any alleged plans to smuggle migrants into the UK.
Police were investigating a suspected Irish people-smuggling ring, the report claimed.
A neighbour of Robinson's parents told Belfast Telegraph that the couple, from Laurelvale in Co Armagh, has flown to England from Northern Ireland to support their son.
It is believed Robinson picked up the trailer two hours before the migrants were found dead.
Temperatures in the refrigerated container get as low as -25C.
The trailer had entered England via a freight ferry service from Zeebrugge, Belgium, into Purfleet, Essex, on the River Thames.
Belgian authorities said the container arrived at Zeebrugge at 2.49pm local time on Tuesday and left the Belgian port the same afternoon.
The ferry docked in Essex just after 12.30am on Wednesday and the lorry left the port in Purfleet about 35 minutes later.
The tractor that hauled it away from the port had come from from Northern Ireland, according to police.
It had "Ireland" emblazoned on the windscreen along with the message "The Ultimate Dream".
The Bulgarian ministry of foreign affairs said the truck was registered in Varna in Bulgaria "under the name of a company owned by an Irish citizen".
Paramedics were alerted first after the bodies were discovered and Essex Police were notified just after 1.40am.
The migrants were found dead in the trailer at the Waterglade Industrial Park in Grays.
The lorry was moved to nearby Tilbury Docks so the bodies can be recovered while preserving the dignity of the victims.
Home Secretary Priti Patel tweeted: "This morning I met officers from Essex Police to express my thanks for their response to the tragic incident in Grays & receive an update on the investigation.
"I stand behind them as they continue their work to establish how this horrific event came to happen."
Belgium's federal prosecutor's office has launched its own investigation to find out where the trailer originated and how long it had been in the country.
The office said it is not yet clear when or where the victims entered the container.
C.RO Ports, which manages the ferry terminals in Zebrugge and Purfleet said it was "deeply saddened" and assisting investigators.
Neighbours told Irish media that Robinson's parents had already travelled to England to support their son.
His father, Mark, is the manager of Laurelvale FC, a local amateur club in the Mid-Ulster Football League.
Paul Berry, a local councillor, said he spoke to Robinson's dad "a couple of times" on Wednesday before the father found out about the arrest on social media.
Mr Berry told the Belfast Telegraph: "When I was talking to him nothing had been confirmed to him that it was Mo. The police or nobody of that nature had informed him."
He stressed Mo Robinson was "innocent until proven guilty".
The councillor told PA: "He had said he had been getting messages via people on social media on what was happening and at that stage it was not confirmed to him or his family that his son had been arrested.
"In the local area the feeling is one of complete shock and hope this is not a true story in terms of his involvement."
Mr Berry, who knows the father well, said the family were "very well respected" in the area.
He added: "The local community is hoping that he (Mo Robinson) has been caught up innocently in this matter but that's in the hands of Essex Police, and we will leave it in their professional hands to try to catch the perpetrators of this."
He said the family had been left upset by the "unwelcome spotlight" the incident had shone on them.
"It was very distressing for the family as they just felt they were captive in their own home," he said.
Wednesday's incident follows warnings from the National Crime Agency and the Border Force of the increased risk of people-smuggling via Belgium.
Security checks for people smuggling are believed to be less stringent at both ports than at Calais and Dover.
