Police have raided three addresses in Northern Ireland and continue to question a man, 25, over the deaths of 39 people, believed to be migrants from China, found in a refrigerated lorry on Wednesday.
The searches in County Armagh on Wednesday night are reportedly linked to the arrest of the driver, named in local reports as Mo Robinson, from Portadown.
A magistrate gave detectives another 24 hours to question the driver.
Police said identifying the victims and working out the circumstances around the deaths of the 38 adults and one child would be a long and complex process.
"We have since confirmed that eight of the deceased are women and 31 are men, and all are believed to be Chinese nationals," Essex Police said.
The force said it was the largest murder investigation it had carried out.
China's Foreign Ministry said its embassy staff in London were heading to the scene "to verify this situation".
Preliminary investigations, in what is the UK's country's largest murder investigation for more than a decade, appear to show the container trailer arrived in the Beligian village of Zeebrugge mid-afternoon on October 22.
It left port the same day for Purfleet in the UK, where it arrived at 12.30am the next day by ferry.
It was collected by the tractor unit of the lorry, which had entered the UK on Sunday, October 20, from Dublin.
Police were called to the scene at Waterglade Industrial Park in Grays, Essex about 1.40am on October 23.
It is not yet clear where the container came from before it entered the port of Zeebrugge or when the people inside died.
Authorities have warned that migrants are making increasingly risky attempts to enter the UK as reports emerged that the travellers might have frozen to death.
"This is an incredibly sensitive and high-profile investigation, and we are working swiftly to gather as full a picture as possible as to how these people lost their lives," said Pippa Mills, deputy chief of Essex Police.
A May 2019 report by the National Crime Agency said: “The past year has seen increasing use of higher risk methods of clandestine entry.
"These include the movement of migrants [including children] into the UK in containers, refrigerated heavy goods vehicles and small boats, at a high risk to life of those migrants smuggled.”
These attempts posed “a significant threat to life", the NCA said.
A 2018 report by the agency said that people smugglers favoured hard-sided, refrigerated lorries.
"Belgium has become a location of greater focus for the activities of organised people smugglers in the past year, where smugglers of various nationalities operate," it said.
"The number of smugglers located there increased after the closure of the migrant camp at Dunkirk in March 2017."
Bulgaria confirmed that the lorry had licence plates issued by the country.
Prime Minister Boyko Borisov said it was registered there in 2017 by an Irish citizen, and had not entered Bulgarian territory since.
"This is an unimaginable tragedy and truly heart-breaking," UK Prime Minister Boris Johnson said.
Mr Johnson said people smugglers "should be hunted down and brought to justice".
The deaths bring back track memories from 2000 when 58 Chinese migrants suffocated in a truck in Dover, England, after a months-long journey from China.