Police have made a fourth arrest over the discovery of 39 bodies in a truck in Essex, but have yet to comment on claims by worried families that Vietnamese citizens are among the dead.
A 48-year-old man from Northern Ireland was arrested at Stansted Airport on suspicion of conspiracy to traffic people and suspicion of manslaughter.
Essex Police earlier said the victims — eight women and 31 men — are all "believed to be Chinese nationals."
However, the Vietnamese Embassy in London said its consular hotline had received calls from distressed citizens saying they feared relatives were on the truck. The embassy said it was in contact with police and other agencies but had yet to receive information.
Deputy Chief Constable Pippa Mills told reporters that the force "will not be commenting" on the nationalities of the victims.
She appealed to anyone with information, including possible illegal immigrants, to come forward.
"Please come forward and speak to us without fear," she said. "I can assure you that your information will be received in the strictest confidence and no criminal action will be taken against you."
In a message to relatives, she said: "I can’t begin to comprehend what some of you must be going through right now. You have my assurance that Essex Police will be working tirelessly to understand the whole picture to this absolute tragedy."
Hoa Nghiem from Human Rights Space, a civic network based in Vietnam, earlier said at least one of the deceased might have been Vietnamese.
Pham Thi Tra My, 26, sent a text message to her mother saying she could not breathe at about the time the truck container was en route from Belgium to Britain, Hoa said.
VietHome, an organisation for the Vietnamese community, said it had received news from 10 families that their loved ones were missing. Hanoi's London embassy was coordinating with British police, the official Vietnam News Agency (VNA) reported.
Earlier, a 38-year-old man and woman from Warrington, Cheshire, were arrested on suspicion of conspiracy to traffic people and on suspicion of 39 counts of manslaughter.
The 25-year-old truck driver, from Northern Ireland, was arrested on suspicion of murder on Wednesday and remains in custody.
Officers raided two homes in Northern Ireland on Thursday as part of an investigation into the deaths, which were discovered in a truck in Grays, a port on the River Thames in Essex, east of London.