Thirty nine Chinese nationals were found dead in a refrigerated trailer in Essex, UK.
Police have been granted an extra 24 hours to question truck driver Mo Robinson, 25, on suspicion of murdering the eight women and 31 men.
Three properties in Northern Ireland have been raided and the National Crime Agency is working to establish if “organized crime groups” were involved.
The truck arrived in Purfleet on the River Thames from Zeebrugge in Belgium.
Ambulance staff discovered the bodies in the container at Waterglade Industrial Park in Grays just after 01:30 local time, on October 23.
The trailer left the port at Purfleet shortly after 01:05.
According to police, the tractor unit – the front part of the truck – entered the UK via Holyhead in Wales on October 20, having travelled from Dublin.
Speaking after a magistrate granted Essex Police more time to question Mo Robinson on October 24, Deputy Chief Constable Pippa Mills said her priority was “preserving the dignity of the 39 people who have died and ensuring that we get answers for their loved ones”.
The truck has been moved to a secure site at Tilbury Docks and police are due to begin the process of moving the bodies to a mortuary at Broomfield Hospital in Chelmsford.
They will be taken by private ambulance so that post-mortem examinations can take place, with the force expecting all the bodies to have been moved by the weekend.
Global Trailer Rentals Ltd confirmed to RTE News that it owned the trailer and said it had leased it on October 15.
The company said it had given Essex Police the details of the person and company they had leased it to.
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”.
China’s ambassador to the UK Liu Xiaoming tweeted that the embassy had read the reports of the deaths “with heavy hearts” and was in close contact with British police.
Police initially suggested the truck 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.
The Belgian Federal Public Prosecutor’s Office said the container arrived in Zeebrugge at 14:29 on October 22 and left the port later that afternoon.
It was not clear when the victims were placed in the container or if this happened in Belgium, a spokesman said.