A murder investigation has been launched after the discovery of 39 bodies in a truck container in Essex, southeast England, early Wednesday morning.
Early indications suggest 38 of the dead are adults and one is a teenager.
A 25-year-old man from Northern Ireland has been arrested on suspicion of murder.
A cordon has been put in place and access to and from the Waterglade Industrial Park, where the truck container was found, remains closed.
Police 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.”
A spokeswoman for police in Bulgaria confirmed they were aware of the incident, but said British officers were leading the investigation.
It is thought the truck will have entered Holyhead, Wales, via Dublin.
Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association (FTA), said: “There is a direct route to Holyhead from Dublin.
“If the lorry came from Bulgaria, getting into Britain via Holyhead is an unorthodox route.
“People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.
“It’s a long way around and it’ll add an extra day to the journey.”
Road Haulage Association chief executive Richard Burnett said the truck could have travelled on a ferry from Cherbourg, France, to Rosslare, Ireland, before driving to Dublin and taking another ferry to Holyhead, North Wales, and then reaching Kent by road.
He told PA: “It’s highly unlikely that if this vehicle has come from Europe that it’s been physically checked.
“Because of the migrant issue at Dover and Calais, you’ve got far more checks that are taking place there. You’ve got heartbeat monitors, you’ve got dogs, you’ve got CO2. Those checks are done as you drive through.
“Cherbourg, because it’s a low volume port, you probably won’t have the same security measures that they have in Coquelles, Calais, for the high number of vehicles that are stepping through there and that’s been one of the main migrant routes historically.
“If this is somebody trying to smuggle a significant number of people through then maybe Cherbourg has been picked because it’s a little easier to get through.”
Burnett told PA the container appeared to be a refrigerated unit and described conditions for anyone inside as “absolutely horrendous.”
Temperatures in such units can be as low as -25C (-13F) if frozen products are being transported, causing humans to “lose their lives pretty quickly,” he said.
He went on: “It’s going to be dark. If the fridge is running it’s going to be incredibly cold.
“The only place to go to the toilet is on board the back of the trailer. You can imagine if they’ve been in there for days then there will be faeces, there will be urine.”
Prime Minister Boris Johnson tweeted: “I’m appalled by this tragic incident in Essex.
“I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened.
“My thoughts are with all those who lost their lives & their loved ones.”
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 said.
“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,” 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.”
The National Crime Agency (NCA) said: “We are aware of this tragic incident which is now the subject of a murder investigation being led by Essex Police and we have deployed NCA officers to assist.
“We are working with partners including Essex Police and Immigration Enforcement to provide specialist support to urgently identify and take action against any organized crime groups who might have played a role in causing these deaths.”
This story has been updated throughout.