A "horrified" lorry driver found the bodies of 39 migrants inside a trailer he was hauling when when he went to check paperwork, a friend claims.
Friends have said the trucker, named locally as 25-year-old Mo Robinson, raised the alarm himself after finding the victims - all Chinese nationals - dead inside the refrigerated container.
Robinson, from Portadown in Northern Ireland, has been arrested on suspicion of murder and was still in custody for questioning on Thursday, Mirror.co.uk reports.
The victims were found dead in Grays, Essex, shortly after the trailer was picked up at a port on the River Thames after arriving from Belgium.
A friend claimed Robinson was innocent, telling MailOnline: "(Mo) wouldn’t have known those people were in the back.
"Apparently he’d parked up at the industrial estate in Essex and had gone to fetch the paperwork from a pocket on the inside of the lorry door.
"When he opened the container up and saw all the dead bodies, he was absolutely horrified - as anyone would be - and called the ambulance service who in turn alerted the police."
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.
Neighbours told Irish media that Robinson's parents had left their home in Laurelvale, Co Armagh, and had already travelled to England to support their son.
Paul Berry, a local councillor who knows the parents, said Robinson's father found out about his son's arrest through social media.
The bodies of 31 men and eight women were discovered inside the container at an industrial estate in Grays in the early hours of Wednesday morning.
It is feared they all froze to death inside the trailer where temperatures are said to get as low as -25C.
Officers have started the grim job of removing the bodies from the lorry and the painstaking task of trying to identify them.
It is believed the stowaways had been inside for more than 12 hours by the time they were found dead.
Police probing one of the UK's biggest murder inquiries continued to question Robinson - whose girlfriend is said to be expecting twins - on Thursday.
Officers have also raided three properties in Northern Ireland.
It is believed Robinson - who called his truck the "Polar Express" - picked up the trailer two hours before the migrants were found dead.
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, entering the UK via Holyhead on Sunday on a ferry from Dublin, according to police.
It had "Ireland" emblazoned on the windscreen along with the message "The Ultimate Dream".
On social media, Robinson had dubbed his cab the "Polar Express".
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 at the Waterglade Industrial Park in Grays, and Essex Police were notified just after 1.40am.
Police moved the lorry to nearby Tilbury Docks so the bodies could be recovered.
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.
Belgian authorities believe the migrants boarded the trailer at a location away from the port in Zeebrugge.
C.RO Ports, which manages the ferry terminals in Zebrugge and Purfleet said it was "deeply saddened" and assisting investigators.
The incident has been compared to Britain's biggest illegal immigrant tragedy in which the bodies of 58 Chinese migrants were found crammed into a tomato truck at the southern port of Dover in 2000.
The vehicle had begun its journey in Zeebrugge.
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.
