The 39 dead bodies have been found in a truck container in Essex were Chinese nationals.
The police who are continuing to question lorry driver Mo Robinson on suspicion of murdering the eight women and 31 men.
Officials in Northern Ireland have raided three properties and the National Crime Agency said it was working to identify “organised crime groups who may have played a part”.
The container arrived in Purfleet on the River Thames from Zeebrugge in Belgium. The truck was found at Waterglade Industrial Park in Grays.
Police say the tractor unit of the truck entered the country via Holyhead in Wales on Sunday, having travelled from Dublin.
The lorry has been moved to a secure site at Tilbury Docks so investigations could be “conducted in peace” and to give the “utmost dignity to those within the trailer”.
Essex Police said it was the largest murder investigation in the force’s history and the victims were all “believed to be Chinese nationals”.
It said formal identification of the 39 people, one of whom is a young adult woman, “could be a lengthy process”.
The Chinese ambassador to the United Kingdom, Liu Xiaoming said was in close contact with with British police.
Police initially suggested the lorry could be from Bulgaria, but later said officers believed it entered the UK from Belgium.
A spokesman for the Bulgarian foreign affairs ministry said the truck was registered in the country under the name of a company owned by an Irish citizen.
It’s still not known when the victims were placed in the container.
Prime Minister Boris Johnson speaking during the Prime Minister’s Questions in Parliament on Wednesday said, “I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones. I’m receiving regular updates. The Home Office will work closely with Essex Police as we establish exactly what has happened.”
Most asylum-seekers wanting to reach the Europe enter via hiding in trucks. The regular practice has also resulted in huge number of deaths in the past as well.
In 2017, 71 people were found dead in an abandoned lorry on an Austrian motorway. Going back further to 2000, 58 Chinese migrants were found suffocated to death in a lorry at Dover.