A refrigerated lorry trailer containing the bodies of 39 people travelled from Zeebrugge in Belgium to Essex, police said.
The vehicle, which is believed to have come from Bulgaria, was found by police at Waterglade Industrial Park in Grays, Essex, early on Wednesday.
Essex Police said 38 adults and one teenager were pronounced dead at the scene.
Police believe the trailer was transported from the Belgian port of Zeebrugge into Purfleet, and docked in the Thurrock area shortly after 12.30am on Wednesday.
The tractor unit of the lorry is believed to have come from Northern Ireland.
There are roll-on, roll-off freight ferry services between Zeebrugge and Purfleet.
Roll-on, roll-off freight ferries are designed to carry vehicles, including cars and lorries.
Police said the lorry driver, a 25-year-old man from Northern Ireland, had been arrested on suspicion of murder.