A lorry driver from Co Armagh was arrested on suspicion of murder after the bodies of 39 people were found in a lorry container in Essex.
The 25-year-old was named locally as Mo Robinson from the Laurelvale area near Portadown.
He was arrested after the lorry was found at Waterglade Industrial Park in Grays shortly before 1.40am yesterday.
Essex Police confirmed this morning that the 31 men and eight women who were found dead were Chinese nationals.
Police have searched three properties in Co Armagh in connection with the murder inquiry.
Police initially believed the lorry may have travelled into Britain via the Dublin-Holyhead ferry crossing on Saturday October 19.
It is now known that the refrigerated trailer travelled from Zeebrugge in Belgium to the English port of Purfleet shortly after 12.30am yesterday.
The front 'tractor unit' of the lorry travelled from Dublin to Holyhead on Sunday October 20.
The lorry and trailer left the port shortly after 1.05am.
Police attended the scene after they a received a call from the Ambulance Service at around 1.40am.
A spokeswoman for the PSNI said it is "supporting Essex Police in its investigation into the murder of 39 people".
A spokesman for Belfast Harbour said it has not been asked to investigate whether the lorry came from its port.
"At this time, Belfast Harbour Police has not been asked to investigate; Essex Police is investigating and leading inquiries," he said.
"We are available to provide any assistance that may be required to support the investigation and our thoughts are with the families of those who have lost their lives."
The lorry is registered in Bulgaria by a company owned by an Irish woman, the Bulgarian foreign ministry said yesterday.
"The Scania truck was registered in Varna (on the east coast) under the name of a company owned by an Irish citizen," a spokesman said.
However, he said it is "highly unlikely" that the victims are Bulgarian.
Bulgarian public broadcaster BNR reported that the truck was registered in Bulgaria on June 19 2017, left the country the next day and has not returned since.
Essex Police had last night not identified the victims or which country they are from.
Deputy Chief Constable Pippa Mills said the process is expected to be "lengthy".
She also said she would not be commenting on the lorry driver's identity.
"This matter has attracted national and international interest and it is absolutely imperative that the operation is conducted with the utmost respect for the 39 people who have lost their lives," she said.
"The lorry and the trailer will be moved to a secure location, Tilbury Docks, so the bodies can be recovered while preserving the dignity of the victims. We are yet to identify them and must manage this sensitively with their families."
The National Crime Agency said it had sent officers to assist and identify any "organised crime groups who may have played a part".
Richard Burnett, chief executive of the Road Haulage Association, said: "This tragedy highlights the danger of migrant gangs people-smuggling on lorries."
He said temperatures in refrigerated units can be as low as minus 25C and described conditions for anyone inside as "absolutely horrendous"
Prime Minister Boris Johnson said it was an "unimaginable tragedy and truly heartbreaking".
Speaking at Prime Minister's Questions in the Commons, he said: "I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones."
The Irish Refugee Council said the bodies represented a needless loss of life.
"It is very likely that countless other, unrecorded lives have been lost as Europe focuses on stemming migration flows," a statement said.
"No person should find themselves in a situation where they feel that their only choice is to pay smugglers to transport them on dangerous and arduous journeys with no guarantees for their safety or how their journey might end.
"Now it is time for self-reflection and outrage at the role EU policies and deals are playing in avoidable loss of human life.
"Thirty-nine people are gone; 39 families will grieve their loss."
Guy Verhofstadt, the European Parliament's Brexit coordinator, last night tweeted: "Free movement of people is not the problem in our union. The real problem is the lack of safe, legal migration routes leading to 39 deaths. Whatever happens next in Brexit: the EU and UK need a common approach to prevent these tragedies from happening."
