A murder investigation has been launched after 39 bodies were found inside a lorry container on an industrial estate.
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 are 38 adults and one teenager, police said.
The lorry is from Bulgaria and entered the British mainland at Holyhead, North Wales, one of the main ports 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.”
Paige Wade was driving past the industrial park at 4.15am on the way home from work.
She told the PA news agency: “I had just finished work while driving up Motherwell Way and all you could see was blue flashing lights, as I was driving past there was police tape to cordon off the start of Eastern Avenue.
“I knew it was serious because of how many police cars and ambulances were there, but the police had parked their cars across the whole access of the road so you couldn’t see anything.
“There’s always lorries around there as they park up there for the night.”
Since you're here ...
We know you don't need a lecture. You wouldn't be here if you didn't care.
Now, more than ever, we need your help to challenge the rightwing press and hold power to account. Please help us survive and thrive.