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 in Belgium.
Ambulance staff discovered the bodies of the 38 adults and one teenager 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.
Councillor Paul Berry said the village of Laurelvale, where the Robinson family live, was in “complete shock”.
He said he had been in contact with Mr Robinson’s father, who had learned of his son’s arrest through social media.
“The local community is hoping that he [Mo Robinson] has been caught up innocently in this matter but that’s in the hands of Essex Police, and we will leave it in their professional hands to try to catch the perpetrators of this,” he said.
The Belgian Federal Public Prosecutor’s Office said it had opened a case which would focus on the organisers and others involved in the transport.
A spokesman said the container arrived in Zeebrugge at 14:29 on Tuesday and left the port later that afternoon before arriving in Purfleet in the early hours of Wednesday.
It was not clear when the victims were placed in the container or if this happened in Belgium, he said.
The lorry was moved to a secure site at Tilbury Docks on Wednesday so the bodies could be “recovered while preserving the dignity of the victims”.
Essex Police said the nationalities of the victims were not immediately known and said formal identification “could be a lengthy process”.
The force earlier 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.
He said it was “highly unlikely” the deceased were Bulgarians.
Shaun Sawyer, Chief Constable of Devon and Cornwall Police and the National Police Chiefs Council lead for modern slavery and human trafficking, said while forces had prevented thousands of deaths, “tragically, for 39 people that didn’t work yesterday”.
He told BBC Radio 4’s Today programme even if there were routes perceived as easier to get through, organised criminals would still exploit people who could not access those.
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”.
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.”
How many migrants have died in transit?
The number of migrants who die in transit has been recorded by the UN since 2014.
Since then, five bodies of suspected migrants have been found in lorries or containers in the UK.
Data was not collected in the same way before the migrant crisis began in 2014, but such deaths are not new.
In 2000, 58 Chinese migrants were found suffocated to death in a lorry at Dover.
In 2015, the bodies of 71 people were found in an abandoned lorry on an Austrian motorway. Police suspected the vehicle was part of a Bulgarian-Hungarian human trafficking operation.
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