The arrested lorry driver has been named locally as Mo Robinson, from the Portadown area of County Armagh
Earlier reported on spillednews.com
The driver of the vehicle has been revealed as Mo Robinson.
Robinson 25, is from Portadown, County Armagh, Northern Ireland, has been arrested on suspicion of murder.
The vehicle was found shortly before 01:40 BST at Waterglade Industrial Park in Grays, police said.
Essex Police said the trailer has been moved to a secure location where the bodies of those inside – a teenager and 38 adults – can be recovered.
Essex Police said the trailer arrived via ferry from Zeebrugge, Belgium, into Purfleet on the River Thames.
The ship docked in the Thurrock area shortly after 00:30, the force said.
It is believed the lorry then left the port about 35 minutes later.
Deputy Chief Constable Pippa Mills said identifying the victims remained a “number one priority”, but was expected to be a “lengthy process”.
The National Crime Agency said it had sent officers to assist and identify any “organised crime groups who may have played a part”.
Richard Burnett, chief executive of the Road Haulage Association, said the trailer appeared to be a refrigerated unit, where temperatures could be as low as -25C.
He described conditions for anyone inside as “absolutely horrendous”.