It comes after Maurice Robinson, 25, of Northern Ireland appeared in court charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration, and money laundering.
He has been remanded in custody and will next appear on November 25 at the Old Bailey in London.
Britain has sent documents to Vietnam to help identify some of the 39 people found dead in a truck as fears mount that most of the dead were Vietnamese.
The eight women and 31 men found on Wednesday in a refrigerated container in Essex, southeast England, were originally identified as Chinese.
Mo Robinson has been arrested on suspicion of murder after 39 bodies were found in a lorry in Essex. Photo / Facebook
But several Vietnamese families have come forward saying they fear their relatives are among the dead.
Officials started collecting DNA samples from families in Nghe An and Ha Tinh, impoverished central provinces where most of the suspected victims came from.
BROTHERS HUNTED BY COPS
The Hughes brothers run a haulage business on the Northern Ireland border with the Republic.
Ronan Hughes is alleged to have leased the refrigerated container from another Irish company, Dublin-based Global Trailer Rentals (GTR).
Armed police seized a blue Scania lorry cab at Dublin Port on Saturday, The Sun reports.
Police forensic officers attend the scene after a truck was found to contain a large number of dead bodies, in Thurrock, South England. Photo / AP
There is no suggestion the brothers were aware of the plot to smuggle migrants using the cab or the container.
Mr Robinson was quizzed for more than 72 hours by police after he picked up the trailer from Purfleet, Essex.
Just 20 minutes later, the grisly discovery of the eight women and 31 men in the back of the deathtrap lorry was made on an industrial estate in Grays.
UK Prime Minister Boris Johnson signed a book of condolence set up in the wake of the horror.
He wrote: "The whole nation, and indeed the world, had been shocked by this tragedy and the cruelty of the fate that has been suffered by innocent people who were hoping for a better life in this country.
Britain's Prime Minister Boris Johnson signs a book of condolence during a visit to Thurrock Council Offices, England. Photo / AP
"We mourn those who lost their lives. Our thoughts are with their families far away. In condemning the callousness of those responsible for this crime, we in the government of the United kingdom resolve to do everything in our power to bring the perpetrators to justice."
Mr Robinson is said to have opened the container to get some paperwork and passed out when he discovered the victims' bodies after they reportedly froze to death.
OTHER ARRESTS
Three other people were also arrested in connection with the tragedy, including haulage boss Thomas Maher and wife Joanna Maher, both 38, from Warrington, Cheshire.
Joanna allegedly owned the Scania truck, which was registered in the Bulgarian port of Varna in 2018, but claims she sold it last year.
The pair are being held on suspicion of conspiracy to traffic people and manslaughter but were released on bail until November 11.
Police leave the home of Joanna and Thomas Maher with an evidence box at Wiltshire close in Warrington, Cheshire. Photo / AP
A 48-year-old man from Northern Ireland was also quizzed on the same charges after being detained at Stansted Airport on Friday. He was released on bail until November 13.
A 23-year-old man suspected of driving the container packed with migrants to Zeebrugge before the doomed journey has appeared in court in Ireland on unrelated charges.
Essex Police said the suspect, who was held at Dublin Port when he arrived on a ferry from France, are liaising with cops in Ireland.
The man is a "person of interest" but was arrested on an unrelated charge following a road accident in Germany.
He was remanded in custody by a court in Dublin on Saturday to next appear on Wednesday.
Essex Police originally said they believed the victims were Chinese nationals – but it's now thought the majority are Vietnamese.
The distraught family of Pham Thi Tra My, 26, are now frantically scrambling for information after paying £30,000 ($60,720) to smuggle her to the UK.
She sent her family chilling messages saying she was "dying" and couldn't breathe as the doomed trailer was on its way to Essex from Belgium.
Pham This Try Ma is thought to be among the victims.
Other Vietnamese nationals – including Nguyen Dinh Luong and Bui Thi Nhung – are feared to be among the dead after their relatives reported them missing.
Detective Chief Inspector Martin Pasmore said the nationality of the victims was not yet known, but the focus is now on the Vietnamese community – although "there may be other nationalities involved".
Bodies are being identified by their fingerprints at Broomfield Hospital, where post-mortem examinations are also taking place.
