UK police say they have arrested the fourth person on trafficking offences and manslaughter suspicions. The arrest took place at London's Stansted airport.
The police added that it would not comment on speculation in the media about nationalities of those 39 people who were found dead inside a truck in Essex earlier this week.
On Friday, Vietnamese media said that the Vietnamese Embassy in London was working with British police to identify one of the victims.
Earlier in the day, a man and a woman, both 38, from Northern Ireland were arrested on suspicion of human trafficking and manslaughter as part of an investigation into the incident in which the bodies of 39 people were found in a truck in Essex earlier this week, British police say.
The arrest took place in Warrington, northern England, according to the Essex police.
Previously, the Chinese Embassy in the UK said that British police were unable to confirm that the 39 people who were found dead inside a truck container had Chinese citizenship.
