British police have launched a murder investigation after the bodies of 39 people were found in the container of a truck that is believed to have come from Bulgaria.

The truck was discovered at an industrial park outside London in the early hours of Wednesday local time, and all 39 people inside were found dead at the scene, police said.

The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.

Police believe the vehicle came from Bulgaria and entered the United Kingdom on Saturday. One of the victims was a teenager and all the rest were adults, they said.

The truck was found at the Waterglade Industrial Park in the town of Grays in the county of Essex, around 15 miles east of central London.

At a news conference Wednesday, Deputy Chief Constable Pippa Mills of Essex police called it a "tragic discovery" and said "the identification of the victims remains our No. 1 priority."

"This is an absolute tragedy and a very sad day for Essex police and in the local community," Mills said. "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."

Prime Minister Boris Johnson tweeted that he was "appalled by this tragic incident," adding that the government would work with police to establish what happened. British Interior Minister Priti Patel tweeted that she was "shocked and saddened by this utterly tragic incident."

Police said they believe the vehicle is from Bulgaria, which shares a border with Turkey and is one of the gateways into the European Union. They believe it came into the U.K. through the Welsh port of Holyhead, raising the possibility it entered the country from Ireland.

Irish police would only say in an emailed statement that it was "monitoring the developing investigation in the U.K. and will provide every assistance possible."

The National Crime Agency — which investigates organized crime including human trafficking — said it has deployed officers to the scene.

The human rights watchdog Amnesty International called it a "a heartbreaking and horrifying" incident.

"People who are forced to take dangerous and sometimes fatal passages to reach the U.K. often do so because current immigration policies and practices deny them safe and legal options," said a statement from Steve Valdez-Symonds, the group's U.K. refugee and migrant rights director.