Detectives are continuing to question four people over the deaths of 39 migrants whose bodies were found inside a lorry in Essex.
A 48-year-old man from Northern Ireland was detained at Stansted Airport on Friday on suspicion of conspiracy to traffic people and manslaughter, Essex Police said.
Officers had earlier arrested a couple, named locally as haulage boss Thomas Maher and his wife Joanna, both 38, of Warrington, on suspicion of 39 counts of manslaughter and people trafficking.
It followed the arrest of the driver of the Scania truck on suspicion of murder on Wednesday, who remains in custody.
On Thursday, detectives were granted more time to question Mo Robinson, 25, from Northern Ireland, after the bodies of eight women and 31 men were found in the refrigerated trailer in an industrial park in Grays in the early hours of Wednesday.
In Belgium, police are hunting the driver who delivered the trailer to Zeebrugge, the port it left before arriving in the UK.
A spokesman from the Belgian prosecutor’s office told the PA news agency: “We’re trying to identify the driver.”
He said Belgian authorities were also working to “track the route of the container” and find anyone responsible for “collaborating with the transport”.
“We would like people to be arrested as soon as possible,” he added.
Essex Police initially believed all of the dead were Chinese nationals, but the force said at a press conference “this is now a developing picture” amid reports several may be Vietnamese.
Deputy Chief Constable Pippa Mills said she would give no more details about the nationalities of the victims until formal identification had taken place.
The BBC said it has been in contact with six Vietnamese families who fear their relatives are among the dead, with some having the smuggling fees repaid.
Relatives of 26-year-old Pham Tra My told the broadcaster they have not been able to contact her since she sent a text on Tuesday night saying she was suffocating.
“I am really, really sorry, Mum and Dad, my trip to a foreign land has failed,” she wrote.
“I am dying, I can’t breathe. I love you very much Mum and Dad. I am sorry, Mother.”
Her family said they had paid £30,000 for her to be smuggled into Britain, which has now been repaid.
Her father, Pham Van Thin, told CNN: “The smugglers said that this was a… safe route, that people would go by airplane, car… if I had known she would go by this route, I would not have let her go.”
In Vietnam, Nguyen Dinh Gia, the father of 20-year-old Nguyen Dinh Luong, feared his son was among the container victims.
He had not spoken to his son since last week when he told his father he was trying to reach the UK by joining a group in Paris.
“He often called home but I haven’t been able to reach him since the last time we talked last week,” Nguyen Dinh Gia told the Associated Press.
“I told him that he could go to anywhere he wants as long as it’s safe. He shouldn’t worry about money, I’ll take care of it.”
He explained his son left Vietnam to work in Russia in 2017 and had since passed through Ukraine, Germany and France.
If the fridge on the hermetically sealed trailer was not running there would be no air coming in, suffocating people inside, according to Richard Burnett, chief executive of the Road Haulage Association.
It is not yet known when the victims entered the trailer, where temperatures can be as low as minus 25C if the fridge is activated, or the exact route it travelled.
Belgian officials said the trailer arrived at Zeebrugge at 2.49pm on Tuesday and left the port the same day en route to Purfleet.
The trailer arrived at Purfleet at around 12.30am on Wednesday, and was picked up by the cab, known as the tractor, which arrived from Northern Ireland via Holyhead in North Wales on Sunday.
The lorry left the port at Purfleet shortly after 1.05am before police were called to the Waterglade Industrial Park on Eastern Avenue in Grays at 1.40am.
China has called for joint efforts to counter human smuggling, while vigils have been held in London and Belfast to pay tribute to the victims.
The Vietnamese embassy in London has said some families have contacted them asking about relatives but it is yet to receive any information from police.