Police in England have arrested a man from Northern Ireland after 39 bodies were found inside a lorry container.
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.
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.”
Richard Burnett, chief executive of the Road Haulage Society in the UK, said “this tragedy highlights the danger of migrant gangs people-smuggling on lorries”.
A cordon has been put in place and access to and from the Waterglade Industrial Park remains closed.
Chief Supt Mariner said: “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.”
A restaurant worker in the industrial estate, who did not want to be named, said the area was cordoned off by police on Wednesday morning.
“There’s a lot of police and forensics,” the Big Blue Food Bus employee told the PA news agency.
“We’ve just seen what has been said online. It’s awful.
“We thought maybe someone had broken into a lorry, but it’s just awful.”
Glen Freeland, from GSF Car Parts which is close to where the bodies were found, said staff could not access the shop on Eastern Avenue on Wednesday morning.
“The manager went to get into work this morning and it was cordoned off and we’ve been moved to a different area,” Mr Freeland said.
“All we know is they found the bodies in the back of a lorry.
“It was a bit of a shock but we’re not 100 per cent sure of what’s happened.”