Police are continuing to question a lorry driver over 39 people found dead in a refrigerated trailer in Essex.
Mo Robinson, 25, from Portadown, County Armagh, was arrested on suspicion of murder on Wednesday by Essex Police.
Officers in Northern Ireland have raided two houses and the National Crime Agency said it was working to identify “organised crime groups who may have played a part”.
The trailer arrived in Purfleet on the River Thames from Zeebrugge, Belgium.
Ambulance staff discovered the bodies in the container at Waterglade Industrial Park in Grays just after 01:30 BST on Wednesday.
The lorry and trailer left the port at Purfleet shortly after 01:05.
Police said the tractor unit – the front part of the lorry – came from Northern Ireland and picked up the trailer from Purfleet.
Eric Van Duyse, a spokesman for the Belgian federal prosecutor’s office, said an investigation was under way.
He added: “We have no idea at the moment how long the lorry spent in Belgium, it could be hours or days, we just don’t know.”
Essex Police Deputy Chief Constable Pippa Mills said the vehicle had been moved to a secure site at Tilbury Docks so the bodies of the 38 adults and one teenager could be “recovered while preserving the dignity of the victims”.
The nationalities of the victims were not immediately known and police said the formal identification “could be a lengthy process”.
Essex Police had earlier suggested that the lorry could be from Bulgaria, but later said they believed the vehicle 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.
He said it was “highly unlikely” the deceased were Bulgarians.
Richard Burnett, chief executive of the Road Haulage Association, said temperatures in refrigerated trailers could be as low as -25C.
He described conditions for anyone inside as “absolutely horrendous”.
Prime Minister Boris Johnson said it was an “unimaginable tragedy and truly heartbreaking”.
Speaking at Prime Minister’s Questions, he 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.”
On Thursday, Thurrock’s Conservative MP Jackie Doyle-Price, said there needed to be an international response.
She said: “We have partnerships in place but those efforts need to be rebooted, this is an international criminal world where many gangs are making lots of money and until states act collectively to tackle that it is going to continue.”
