Police found the bodies of 39 people inside a truck container believed to have come from Bulgaria at an industrial estate to the east of London on Wednesday.

The truck was thought to have entered the U.K. at the seaside town of Holyhead in Wales on Saturday and to have originally started its journey in Bulgaria. The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder, police said.

“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 Supt. 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.”

Police made the grim discovery in the early hours of Wednesday at the Waterglade Industrial Park in Grays, about 32 kilometres from central London. The 39 victims, 38 adults and one teenager, were all pronounced dead at the scene.

U.K. Prime Minister Boris Johnson said he was “appalled by this tragic incident in Essex.”

“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 & their loved ones.”

U.K.Home Secretary Priti Patel expressed her sadness over this “this utterly tragic incident” on twitter. 

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.