Video report by ITV News Correspondent Dan Rivers
Police are continuing to question four suspects over the deaths of 39 people found inside a lorry in Essex.
Officers detained a 48-year-old man from Northern Ireland at Stansted Airport on conspiracy to traffic people and on suspicion of manslaughter on Friday.
A couple named locally as haulage boss Thomas Maher and his wife Joanna, both 38, were arrested on suspicion of 39 counts of manslaughter and people trafficking on Friday.
The driver of the Scania truck, named locally as 25-year-old Mo Robinson, from Northern Ireland, also remains in custody after he was held on suspicion of murder on Wednesday.
The bodies of eight women and 31 men were discovered in a refrigerated trailer in Grays in the early hours of Wednesday.
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”.
Deputy Chief Constable Pippa Mills said she would give no more details about the nationalities of the victims until formal identification had taken place.
Post-mortem examinations are due to begin as the bodies are moved by private ambulance with a police escort from the port of Tilbury to Broomfield Hospital in Chelmsford, Ms Mills said.
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.
“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 brother said: "My sister went missing on October 23 on the way from Vietnam to the UK and we couldn’t contact her. We are concerned she may be in that trailer.
"We are asking the British police to help investigate so that my sister can be returned to the family."
Nguyen Dinh Luong, 20, is also feared to be among the dead, with his brother telling the broadcaster Dinh Luong left Vietnam a year ago before spending time in France.
The Vietnamese Embassy said staff will arrange a visit to site in a bid to identify victims after Vietnamese families said their loved ones may be among the deceased.
It continued: "The Embassy has not yet received any official confirmation from the British relevant agencies. The Embassy has continued to follow the event and worked closely with relevant British agencies to accelerate the investigation process."
Ms Mills said at a press conference on Friday evening: “I can confirm we have officers working around the clock and we have now arrested a fourth person.
“A 48-year-old man from Northern Ireland was arrested at Stansted Airport earlier today on suspicion of conspiracy to traffic people and on suspicion of manslaughter.
“It follows the arrests of a 38-year-old man and a 38-year-old woman from Warrington during warrants executed in Cheshire overnight. They are currently in custody also on suspicion of conspiracy to traffic people and on suspicion of manslaughter.
“A 25-year-old man arrested on Wednesday on suspicion of murder remains in custody.”
Police officers could be seen at the Mahers’ four-bedroom house in Warrington, which had two grey Range Rovers with personalised number plates and a white Chevrolet sports car parked on the drive.
The previous day the couple, originally from southern Ireland, claimed to have sold the Bulgarian-registered Scania lorry cab, which picked up the trailer at the port of Purfleet, to a company in Ireland.
Mrs Maher, who is reportedly the last known owner of the vehicle, told MailOnline: “We did own it but sold it 13 months ago.”
Locals said Mr Maher is the owner of a haulage firm and his wife works as a hairdresser.
They said they have three teenage children, two boys aged 18 and 11 and a 15-year-old girl, and enjoy numerous foreign holidays, including trips to Mexico and long cruises.
Meanwhile, people who live near Purfleet, said illegal migrants were a familiar sight.
Lee Tubby, 45, who lives opposite the port, said he has seen people “climbing out the top and out the back” of lorries and cutting the plastic roof covering to climb through.
“We’ve had people just come out of the port knocking on the door asking for shoes, asking for water,” he said.
It is not yet known when the victims entered the sealed refrigerated trailer, where temperatures can be as low as minus 25C, 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 a 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.
Irish company Global Trailer Rentals Ltd (GTR) said the trailer had been leased on October 15 from its rentals yard in Co Monaghan, in the Republic of Ireland, at a rate of 275 euro (£237) a week.
It said it provided police with information about the person and company that leased the trailer, as well as offering to make tracking data available.
Three addresses have been searched in Northern Ireland as part of the probe, while warrants were also carried out in Cheshire.
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.