The discovery was made in the early hours after emergency services were alerted to people in the container on a gritty industrial site in Grays, about 20 miles from central London.
Police said the driver of the truck, a 25-year-old man from Northern Ireland, had been arrested on suspicion of murder. Prime Minister Boris Johnson said he was appalled.
“I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened,” Johnson said on Twitter. “My thoughts are with all those who lost their lives & their loved ones.”
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.
— Boris Johnson (@BorisJohnson) October 23, 2019
Police said the truck was thought to have entered Britain at Holyhead, a North Wales port that is a major entry point for traffic from Ireland, on Saturday and to have originally started its journey in Bulgaria.
The victims, 38 adults and one teenager, were all pronounced dead at the scene after they were found at the Waterglade Industrial Park, not far from docks on the River Thames.
Bulgaria’s foreign ministry said it could not confirm at this stage whether the truck had started its journey from the country.
“We are still checking the information published in the British media and we’re contacting the authorities,” foreign ministry spokeswoman Tsvetana Krasteva said.
Police officers in forensic suits were on Wednesday inspecting a large white container on a red truck next to warehouses at the site. Police had sealed off the surrounding area of the industrial estate with large green barriers as they carried out their investigation.
“This is a tragic incident where a large number of people have lost their lives. We are in the process of identifying the victims, however, I anticipate that this could be a lengthy process,” said Chief Superintendent Andrew Mariner from Essex Police.
“We have arrested the lorry driver in connection with the incident who remains in police custody as our inquiries continue,” 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,” he continued.
For years, illegal immigrants have attempted to reach Britain stowed away in the back of trucks, often seeking to reach the United Kingdom from the European mainland.
In Britain’s biggest illegal immigrant tragedy in 2000, British customs officials found the bodies of 58 Chinese people crammed into a tomato truck at the southern port of Dover.
News of the grisly discovery shocked British lawmakers; the local Member of Parliament speculated that the incident was linked to human trafficking. Police have not confirmed that claim.
Jackie Doyle-Price, who is the Conservative lawmaker for Thurrock, where the bodies were found, wrote on Twitter: “Sickening news of 39 people found dead in a container in Grays.
“People trafficking is a vile and dangerous business. This is a big investigation for Essex Police. Let’s hope they bring these murderers to justice.”
Sickening news of 39 people found dead in a container in Grays. People trafficking is a vile and dangerous business. This is a big investigation for @EssexPoliceUK Lets hope they bring these murderers to justice
— Jackie Doyle-Price (@JackieDP) October 23, 2019
The UK Home Secretary, Priti Patel, wrote on Twitter that she was “shocked and saddened” by what she called the “utterly tragic incident.”
Lwmaker Brandon Lewis also called the news “tragic” on Twitter.
“Such tragic news on loss of life,” Lewis, a junior interior minister, wrote.
“Thoughts are with victims, their families & friends. Police are working to ensure those responsible are brought to justice.”
Such tragic news on loss of life. Thoughts are with victims, their families & friends. Police are working to ensure those responsible are brought to justice.
— Brandon Lewis (@BrandonLewis) October 23, 2019
Jeremy Corbyn, the leader of the opposition Labour Party, said the latest deaths were an unbelievable human tragedy that needed answers.
“Can we just think for a moment of what it must have been like for those 39 people, obviously in a desperate and dangerous situation, for their lives to end, suffocated to death in a container,” he said.
By Michael Holden
The CNN Wire and Lorenz Duchamps contributed to this report.