Officers stand guard at the scene in Grays this morning
Essex Police previously said the vehicle entered the UK via Holyhead in north Wales.
But they have now said that is not the case. The lorry’s cab is believed to have originated in Northern Ireland.
Emergency services made the horrific discovery at around 1.40am today in Eastern Avenue, on the Waterglade Industrial Park.
The bodies, believed to be of 38 adults and one teenager, were found in the back of the vehicle.
The driver of the lorry, a 25-year-old man from Northern Ireland, remains in custody having been arrested on suspicion of murder.
Police say the lorry arrived at Purfleet shortly after 12.30am today, having travelled from Zeebrugge in Belgium.
It left the port shortly after 1.05am before the discovery was made a short while later.
In a statement, Essex Police said: “A cordon has remained in place at Eastern Avenue for most of the day and this remains in place.
“We thank everyone in the area, especially residents and local businesses, for their patience and compassion throughout the day.
“This will be a lengthy and complex investigation and we continue to work with local partners and international authorities to gather vital intelligence and identify those who have sadly died.
“We urge anyone with information to contact our Major Investigation Team on 101 or visit our website.”
Tags: Purfleet, Grays, Thurrock, Lakeside, West Thurrock