Container deaths: Four Irish people now being held by police
Deputy chief constable Pippa Mills from Essex police said a fourth arrest has been made today, a 48-year-old man from Northern Ireland was arrested at Stansted Airport, on suspicion of trafficking people and manslaughter.
Deputy Chief Constable Mills urged the media not to speculate the nationality of the victims, stating officers "owed" it to families to get the investigation right.
This was a "fast moving" investigation she added, as it became clear the focus is increasingly being made on Ireland.
The driver of the truck, named locally as 25-year-old Mo Robinson, from Northern Ireland, remains in custody after he was held on suspicion of murder on Wednesday.
An Irish couple, both 38, remain in custody on suspicion of 39 counts of manslaughter and people trafficking after they were arrested on Friday. The couple were originally from the midlands.
Essex Police initially believed all of the victims were Chinese nationals, but the force said at a press conference "this is now a developing picture".
Deputy Chief Constable Mills said the force would give no more details about the nationalities of the victims until formal identification had taken place.
She said: "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."
The BBC said it has been in contact with six Vietnamese families who fear their relatives are among the victims, with some having the smuggling fees repaid.
Relatives of 26-year-old Pham Tra My told the BBC 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 (€35,000) for her to be smuggled into Britain, which has now been repaid.
Her brother told the broadcaster: "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.
Additional reporting by PA