By Amy Gunia and Rachael Bunyan
Updated: October 23, 2019 12:09 PM ET

A murder investigation is under way after 39 bodies were discovered on Wednesday in a truck parked on an industrial estate in southern England.

Police were called by paramedics in the early hours of Wednesday morning after the bodies were discovered in the town of Grays, Essex.

“This is a tragic incident where a large number of people have lost their lives. Our inquiries are ongoing to establish what has happened,” Essex police chief superintendent Andrew Mariner said in a statement. Police said that the 39 victims include one teenager, and that they are still working to identify the bodies. “I anticipate that this could be a lengthy process,” the superintendent told media.

Police believe the truck traveled from Zeebrugge, Belgium and entered the United Kingdom via the port of Purfleet in Essex, via Zeebrugge in Belgium. (Police had earlier said the truck entered the U.K. through the town of Holyhead in Wales, but have corrected its journey after “further enquiries.”) The truck docked in the Thurrock area of Grays shortly after 12:30am this morning, police added.

The Associated Press has reported that the truck’s license plate is from Bulgaria; its tractor unit is believed to have originated in Northern Ireland. Police say that the driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder and remains in police custody.

“This is an absolute tragedy,” Shoaib Khan, a U.K. human rights lawyer, tells TIME. “While it remains unclear who these people were, they were clearly desperate to enter the U.K., even if it meant risking their lives.

“While the Prime Minister, Home Secretary and others will express sadness and promise investigations… this is the direct result of the British government closing down legitimate, reasonable ways to enter the U.K.,” Khan continued. “Unless humane laws and policies are introduced, such incidents will continue and the most helpless of people will continue to pay with their lives.”

Boris Johnson, the U.K. Prime Minister, expressed sadness during Prime Minister’s Questions in the House of Commons today. “This is an unimaginable tragedy and truly heart-breaking,” he said. “I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones.”

“I’m shocked and saddened by this utterly tragic incident in Grays. My heart goes out to all those affected,” the U.K. Home Secretary Priti Patel, who oversees the U.K.’s immigration policies, said in a statement.

“Essex Police have arrested an individual and we must give them the space to conduct their investigations. Home Office immigration officials will be working closely alongside them to establish how this horrific event came to happen.”

While authorities have not yet offered any details as to why the victims were in the vehicle, there have been several cases of migrants dying while being smuggled across borders in recent years. In 2018, a Hungarian court convicted a group of smugglers for their roles in the trafficking and deaths of 71 migrants whose bodies were found in a truck abandoned next to a highway in Austria in 2015.

In 2017, emergency responders found dozens of undocumented migrants in distress in a semi-trailer in a Walmart parking lot in San Antonio, Texas. Nine people died.

Write to Amy Gunia at amy.gunia@time.com.

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