U.K. police have launched a murder investigation after 39 dead bodies were discovered in a tractor-trailer about 20 miles east of London, police said Wednesday, Oct. 23.

Emergency services were quickly deployed to an industrial park in the town of Grays in Essex, southeast England, at around 1:40 a.m. local time when a truck’s container was discovered to have people inside. Thirty-nine people, one of whom was a teenager, were found dead at the scene.

A 25-year-old man from Northern Ireland has been arrested on suspicion of murder.

“This is a tragic incident where a large number of people have lost their lives,” Essex Police Chief Superintendent Andrew Mariner said in a statement. “Our inquiries 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.”

The trailer reached the U.K. from the Belgian coastal town of Zeebrugge into the town of Purfleet, arriving at around 12:30 a.m. Wednesday, police said. The tractor unit is believed to have moved from Northern Ireland to Purfleet. The tractor and trailer unit then arrived at the industrial park in Grays, where it was found by emergency services shortly after.

U.K. Prime Minister Boris Johnson addressed the tragedy on Twitter, writing, “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.”

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