The 39 people found dead in the back of a lorry in Essex were Chinese nationals, sources said.
It is understood there were men and women discovered in the refrigerated trailer in Grays, Essex, on Wednesday.
Police are continuing to question a 25-year-old man from Northern Ireland on suspicion of murder.
The trailer containing the victims – including a teenager – arrived at Purfleet from Zeebrugge in Belgium at around 12.30am on Wednesday, and the front section to which it was attached, known as the tractor, came from Northern Ireland.
The lorry and trailer left the port at Purfleet shortly after 1.05am and officers were called around 30 minutes later after ambulance staff made the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.
The driver of the lorry has been named locally as Mo Robinson, from Portadown. Councillor Paul Berry said the village of Laurelvale, where the Robinson family live, was in “complete shock”.
On Wednesday, police searched two addresses in Northern Ireland as part of the investigation.
The deaths follow warnings from the National Crime Agency (NCA) and the Border Force of the increased risk of people-smuggling via Belgium and into quieter ports such as Purfleet.
The NCA previously said it had a “greater focus” on rising smuggler numbers in Belgium after the closure of a migrant camp, and a Border Force assessment highlighted Zeebrugge as being among “key ports of embarkation for clandestine arrivals”.
The NCA has also warned that criminal networks are suspected to have started targeting quieter ports on the east and south coasts of the UK as well as the main Channel crossing between Calais and Dover.