Police in southeastern England launched a murder investigation on Wednesday morning after discovering 39 bodies in a box truck driven by an as-yet unnamed 25-year-old man from Northern Ireland. The driver has been taken into custody on suspicion of murder, but few other details have been made available thus far.

According to police, the truck was believed to have originated in Bulgaria, which is a member of the European Union along with Britain for the time being. However, it’s common for European trucking companies to be based in Bulgaria and use it for registration purposes due to its lower costs when compared to other nations on the continent. Indeed, Bulgarian Prime Minister Boyko Borissov has already claimed that the truck has not been in his nation since it was registered to an Irish woman in 2017.

Records show that the truck entered the U.K. via the Welsh ferry port of Holyhead on Saturday. The truck was next seen on Wednesday in the Waterglade Industrial Park in Grays, which is a small town east of London, where police discovered the bodies inside. Of the 39 bodies, 38 are said to have been adults, with one other thought to have been a teenager.

“This is an absolute tragedy and a very sad day for Essex police and in the local community,” Deputy Chief Constable Pippa Mills of Essex Police told a news conference. “We will continue to work alongside many other partner agencies to find out what led to these deaths. I’d like to appeal for anyone who has any information to contact my offices.”

With so few details to work with and a suspect in custody, some of wondered if police may have stumbled across a spree or serial killer that had been operating throughout Europe, but the details of the case that have made their way to the public thus far seem more reminiscent of past tragedies tied to human trafficking, rather than intentional murder. Four years ago, 71 people that had fled Afghanistan, Syria and Iraq were found dead in a box truck in Austria, and in 2000, British authorities found the bodies of 58 others smuggled in from China aboard a tomato truck.

The U.K.’s Road Haulage Association released a statement that suggested the incident was tied to human trafficking, while offering some details that the police did not disclose.

“Our thoughts are with the families of those who have lost their lives, but whatever the circumstances of this tragedy it highlights the danger of migrant gangs people-smuggling on lorries,” a statement from the association’s chief executive, Richard Burnett, reads. Burnett reportedly also told local news outlets that the box truck was a refrigerated unit and that the cooling elements were turned on with the people still inside — potentially freezing them to death.