Updated at 3:15 p.m. ET
British police arrested a 48-year-old man at Stansted Airport Friday as part of an investigation into the deaths of 39 people who were discovered in a trucking container in southeastern England. He is being held on suspicion of conspiracy to traffic people and on suspicion of manslaughter.
It was the third arrest related to the case Friday. Earlier, authorities detained a 38-year-old man and a 38-year-old woman from Warrington, a town west of Manchester, on suspicion of conspiracy to traffic people and on suspicion of 39 counts of manslaughter.
Authorities also announced that the first post-mortem examinations of the bodies will begin Friday. Police say ambulances began moving victims from the container at a port in Essex to mortuary sites on Thursday.
Essex Police discovered the container early Wednesday at an industrial park east of London. The truck's driver, a 25-year-old man from Northern Ireland, was arrested and remains in custody on suspicion of murder.
British authorities said Thursday they believe the deceased were Chinese nationals, something the Chinese Embassy in the U.K. says it has not yet verified.
In a new development, several Vietnamese families say they fear their missing relatives who were trying to emigrate may have died in the container.
The Vietnamese Embassy in London says families have asked for its help early Friday to "verify whether their family members were among the victims."
In a statement emailed to NPR, the embassy said it had already contacted Essex police to ask about the identification of the victims — and that it has shared information from those families with local authorities and Britain's National Crime Agency.
One family told the BBC that they have not heard from their daughter since Tuesday, when she texted them saying she could not breathe.
The British Embassy in Vietnam addressed the growing concerns Friday, saying the victims' nationalities haven't been confirmed.
Deputy Chief Constable Pippa Mills of the Essex police said the force will not comment on speculation about the nationalities of the deceased.
"We gave an initial steer on Thursday on nationality however this is now a developing picture," Mills said. "As such I will not be drawn on any further detail until formal identification processes approved by Her Majesty's Coroner have taken place."
On Thursday, Essex police released more details about how the truck entered the U.K., saying the truck and the container entered the country at separate times.
Police said the truck traveled from Ireland into Wales on Sunday. Meanwhile, the trailer traveled from the Belgian port of Zeebrugge to an Essex port on the Thames River, arriving just after midnight local time Wednesday morning. The two left the port together less than an hour later, according to authorities.
Officials say eight of the dead were women, and 31 were men. The police said 38 are believed to be adults and one is a young adult woman - earlier police reports had suggested she was a teenager.
Police also confirmed that three properties in County Armagh, Ireland, have been searched in connection with their investigation.
Essex Police Chief Constable BJ Harrington released a statement Thursday, saying the investigation is one of the largest in the history of the region. He also expressed sympathy for the victims' families.
The U.K. has been a common destination for Chinese migrants, and similarly tragic incidents have occurred in the past.
In 2000, the bodies of 58 Chinese nationals were found suffocated in a trailer at the English port of Dover. The driver of the Dutch-registered truck was eventually sentenced to 14 years in prison.
Paolo Zialcita is an intern on NPR's News Desk.