The revelations come as the police in the UK continued to question lorry driver Mo Robinson, 25, who was arrested on suspicion of murder and as Belgian authorities have also released details about the trailer which was found early Wednesday at Waterglade Industrial Park in Grays.
It had arrived in Purfleet on the River Thames from Zeebrugge in Belgium, reports the BBC.
Officers in Northern Ireland have also raided two houses and the National Crime Agency said it was working to identify "organised crime groups who may have played a part".
Police said the tractor unit, the front part of the lorry, came from Northern Ireland and picked up the trailer from Purfleet.
According to a report in The Daily Mail, the police were focussing on whether Robinson knew he was picking up immigrants from Purfleet docks on Tuesday night.
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.
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 formal identification process of the 38 adults and one teenager "could be a lengthy process".
The stowaways slowly froze to death in "absolutely horrendous" conditions after they tried to reach the UK on a cargo ferry from Zeebrugge in Belgium.
The migrants were huddled inside the super-chilled sound-proofed container for at least 15 hours by the time the door was opened by Robinson on Wednesday.
UK Prime Minister Boris Johnson said it was an "unimaginable tragedy and truly heartbreaking".
A vigil will be held at 6 p.m. on Thursday outside the Home Office to "call for urgent action to ensure safe passage" for people fleeing war and poverty.
( With inputs from IANS )