Eight women and 31 men were found dead in Grays, Essex, on Wednesday. Police arrested a 25-year-old man from Northern Ireland, named locally as Mo Robinson, on suspicion of murder.

On Wednesday, police searched three properties in Northern Ireland as part of the investigation.

The force said in a statement on Thursday the case was the largest murder investigation in the force's history.

The discovery echoes one in 2000 when the bodies of 58 Chinese illegal immigrants who had paid a criminal gang to be smuggled into the UK were found in a sealed, airless container in Dover.

The trailer containing the 39 dead, including a teenager, arrived at Purfleet from Zeebrugge in Belgium about 12.30am on Wednesday and the front section to which it was attached, known as the tractor, came from Northern Ireland.

The truck and trailer left the port at Purfleet shortly after 1.05am and officers were called about 30 minutes later after ambulance staff made the discovery at Waterglade Industrial Park in Grays.

The driver is from Portadown, County Armagh. Councillor Paul Berry said the village of Laurelvale, where the Robinson family live, was in "complete shock".

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.

The vehicle has been moved to a secure site at nearby Tilbury Docks where the bodies can be recovered and further forensic work undertaken.

"Each of the 39 people must undergo a full coroner's process to establish a cause of death before we move on to attempting to identify each individual within the trailer," the police statement said.

"This will be a substantial operation and, at this stage, we cannot estimate how long these procedures will take."

The National Crime Agency said it was assisting the investigation and working to "urgently identify and take action against any organised crime groups who have played a role in causing these deaths."

Shaun Sawyer, the national spokesman for British police on human trafficking, said many thousands of people were seeking to come to the United Kingdom.

While they were able to rescue many of those smuggled into the country, Britain was perceived by organised crime as a potentially easy target for traffickers.

"You can't turn the United Kingdom into a fortress. We have to accept that we have permeable borders," he told BBC radio.

Home Secretary Priti Patel had a meeting with Essex police on Thursday morning to receive an update on the investigation.

Australian Associated Press