Truck, which is said to have started its journey in Bulgaria, apparently entered the UK at Holyhead, a North Wales port.
British police now believe a container containing 39 bodies on a freight truck had arrived into the eastern port of Purfleet, having travelled from Zeebrugge in Belgium. Investigators had initially believed it had been driven from Bulgaria.
Chief Superintendent Andrew Mariner said on Wednesday that authorities were in the process of trying to identify the deceased; 38 adults and one teenager.
Police were alerted by the ambulance service shortly before 1:40am local time (00:40GMT) following the discovery of a truck container with people inside at the Waterglade Industrial Park, which houses mostly courier and logistics companies in Grays, east of the British capital.
The 25-year-old driver, from Northern Ireland, was arrested on suspicion of murder, the police said.
UK Prime Minister Boris Johnson said he was “appalled by this tragic incident”.
“I am receiving regular updates from the Home Office and will work closely with Essex Police as we establish exactly what has happened,” he said. “My thoughts are with all those who lost their lives and their loved ones.”
Bulgarian authorities said they had received information about the vehicle’s registration from British police.
“The Scania truck was registered in the [Bulgarian] city of Varna under the name of a company owned by an Irish citizen,” Bulgaria’s foreign ministry said.
Bulgarian public broadcaster BNR, citing unnamed sources, said the truck, which was registered in Bulgaria on June 19, 2017, left the Balkan country the next day and had not returned since.
British police believe the lorry started its most recent journey in Northern Ireland, with the container having crossed the English Channel last night.
“My understanding is that the corpses are still inside the truck,” said Al Jazeera’s Paul Brennan, reporting from Grays, around 20 miles east of central London. “But now the forensics experts have done their work, the truck is going to be removed to a more sensitive and forensically secure location.”
“A murder inquiry has begun,” he said, “but everybody’s thoughts will be with the bereaved relatives of the deceased.”
It was important to remember, Brennan added, that truck drivers often pick up their cargo trailers as sealed units and may not have any knowledge of what was inside.
“But the police will certainly want to know what the driver knows,” he said.
Essex Police said the people were pronounced dead at the scene. “This is a tragic incident where a large number of people have lost their lives,” the service said in a statement.
A cordon has been put in place and access to and from the Waterglade Industrial Park remains closed.
“The cordon around the scene is expected to be in place for quite some time,” Brennan said.
“The police are saying it will take a lengthy time to identify the victims,” he added.
Essex Police appealed for anyone with any information about the lorry’s route to get in touch via their website.
“The identification of victims remains our top priority,” said Pippa Mills, Essex Police’s deputy chief constable.
Jackie Doyle-Price, the local Conservative MP, said “people trafficking is a vile and dangerous business”, adding: “Let’s hope they bring these murderers to justice.”
The tragedy bears similarities to the worst of its kind in the UK, when the bodies of 58 Chinese people were found in a container at Dover, Kent, in 2000.
Seven men were jailed by a Dutch court for their role in the human-smuggling operation that led to the young people suffocating, and the Dutch lorry driver was jailed for 14 years.