Police in the United Kingdom discovered 39 dead bodies in the back of a truck in Essex, police said.

The gruesome discovery was made at Waterglade Industrial Park in the town of Grays at about 1:40 a.m. Wednesday, according to Essex police.

“We are in the process of identifying the victims, however, I anticipate that this could be a lengthy process,” Chief Superintendent Andrew Mariner said in a statement.

“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.”

All 39 people, including at least one teenager, were pronounced dead at the scene.

The truck driver, a 25-year-old man from Northern Ireland, has been arrested.

“This is a tragic incident where a large number of people have lost their lives,” Mariner added. “Our enquiries are ongoing to establish what has happened.”