A lorry in which 39 dead bodies were found had travelled from the Belgian village of Zeebrugge, police have said.
It is believed the vehicle - which is registered in Bulgaria - travelled through the Purfleet area of Essex and docked in Thurrock, where it was discovered in the early hours of this morning.
The driver of the lorry, a 25-year-old man from Northern Ireland, remains in custody having been arrested on suspicion of murder.
Essex Police said it would be a "lengthy and complex investigation".
In a statement, the force said: "We were called shortly after 1.40am by our colleagues in the East of England Ambulance Service to reports that, sadly, 39 people had been discovered dead in the container of a lorry at the Waterglade Industrial Park in Eastern Avenue.
Of the bodies, 38 were adults and one was a teenager
"Originally, we reported that the lorry had travelled into the country through Holyhead on Saturday 19 October.
"After further enquiries, we now believe that the trailer travelled from Zeebrugge into Purfleet, and docked in the Thurrock area shortly after 12:30am this morning. The tractor unit of the lorry is believed to have originated in Northern Ireland."
In the wake of the discovery, the boss of a haulage trade association warned that migrant smuggling gangs were a "massive issue" for lorry drivers.
Richard Burnett, chief executive of the Road Haulage Association, said drivers are targeted by the groups "week in, week out".
"Drivers are facing challenges from smugglers and from gangs continuously. They have to be very careful about where they park up, they have to be very careful about checking seals on their trailers to make sure nobody has broken in," he said.
"There's a set of rules laid out by the Home Office to ensure that hauliers and drivers are checking to make sure they haven't got any migrants on board.
"This has been a long term issue."