Murder investigation launched after 39 bodies found in shipping container in Essex
The victims, found at the Waterglade Industrial Park in Grays, included one teenager, Essex Police said.
A 25-year-old man from Northern Ireland has been arrested on suspicion of murder.
Early indications suggest there 38 are adults and one teenager, police said.
The lorry is from Bulgaria and entered the country at Holyhead, North Wales, one of the main port for ferries from Ireland.
Chief Superintendent Andrew Mariner said: "This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process.
"We believe the lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate.
"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue.
"I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible.
"We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally."