A 48-year-old man from Northern Ireland has become the fourth person to be arrested in relation to the deaths of 39 migrants in a truck container in Essex, said the UK police.
He was arrested at Stansted airport earlier on Friday on suspicion of conspiracy to traffic people and on suspicion of manslaughter.
It comes as a man and a woman, both aged 38 and from Warrington, Cheshire, were arrested on suspicion of 39 counts of manslaughter and people trafficking on Friday.
Post-mortem examinations are due to be carried out on the bodies of some of the 39 people found dead in a lorry container two days ago.
A Vietnamese woman is feared to be among 39 migrants found dead in a lorry in Essex.
The family of Pham Tra My, 26, told the BBC they had paid £30,000 for her to be smuggled to Britain but have not been able to contact her since she sent a text on Tuesday night saying she was suffocating.
She sent a series of messages telling her mother she loved her and was "dying because she couldn't breathe".
He last message was sent just two hours before the truck was found.
Pham Thi Tra told her mother: "I'm sorry Mum. My journey abroad hasn't succeeded. Mum, I love you so much. I'm dying because I can't breathe."
Her brother said she had told them not to contact her because "the organisers" did not allow her to receive calls.
It's thought three people in the container may have come from Vietnam. Essex police have said they believe the victims to be Chinese nationals, however, they said identification was ongoing and that could change.
A statement from the Vietnamese Embassy in London said: "Right after the incident happened, on the 24 of October, the Embassy of Vietnam in London has contacted Essex police in order to inquire the identification of the victims.
"Early morning of the 25 of October, via the hot lines of consular protection, the Embassy received requests from some Vietnamese families asking for the Embassy's help (to) verify whether their family members were among the victims.
"Upon these requests, the Embassy has contacted and provided the detailed information related to suspected victims to Essex Police, the Home Office, Foreign & Commonwealth Office, Diplomatic Protection Liaison of Metropolitan Police and National Crime Agency to acquire the relevant information and arrange a site visit to Essex for the Embassy's representatives.
"As of 17:00 of the 25 of October, 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."
An international investigation is under way as post-mortem examinations are due to begin on the bodies found in a refrigerated trailer in Grays in the early hours of Wednesday.
A convoy of funeral directors vehicles enter the Port of Tilbury to collect the bodies of the 39 people found inside the to transport them to Broomfield Hospital in Chelmsford.
The driver of the lorry, Mo Robinson, from Portadown, remains in police custody after Essex Police was granted extra time to question him.
China has called for joint efforts to counter human smuggling and also cast doubt on the nationality of the victims.
An Essex spokesman said: "Formal identification will then follow the coronial process and will be a lengthy but crucial part of this investigation.
"As our investigations continue, the picture may change regarding identification, and we will continue to provide updates when appropriate under the direction of HM Coroner."
Vigils have been held in London and Belfast to pay tribute to the victims.