A bus full of tourists crashed and caught fire on a highway in western Bulgaria, killing at least 45 people, including 12 children, in the early hours of Tuesday, authorities said.

Tourists from neighboring North Macedonia were returning home from a trip to Turkey before their bus went up in flames in what Bulgarian news media described as one of the deadliest bus accidents in the Balkan country’s recent history.

Seven people, one of them a teenage girl, jumped from the flaming vehicle and survived. They suffered burns but were in stable condition, according to the head of the burn clinic at a hospital treating them in the capital, Sofia.

The bodies trapped inside the bus turned to charcoal, Interior Minister Boyko Rashkov told reporters, which made it more difficult to identify the victims.

“The picture is horrifying,” he said. “I have never seen anything like that.”

Flames enveloped the bus near the village of Bosnek as plumes of smoke rose above it, photos on local media showed. Firefighters and police officers later inspected the scorched shell of the vehicle in the middle of the Struma highway, which was closed Tuesday morning.

“This is a huge tragedy,” said Bulgaria’s caretaker prime minister, Stefan Yanev, who went to the highway after the crash, promising a day of mourning in memory of the victims and a swift investigation.

While the cause remains unclear, Bulgarian officials said the bus, which had North Macedonian license plates, appeared to have hit a barrier on the highway in the predawn hours, either before or after it caught fire.

The head of the country’s National Investigation Service, Borislav Sarafov, also told reporters that authorities were looking into “human error” or “technical malfunction” as potential causes of the accident.

North Macedonian Prime Minister Zoran Zaev, who met with survivors, said one of them told him that passengers were sleeping before what sounded like an explosion woke them up. “Seven people managed to break a window and escape,” he told BTV. The Bulgarian television channel reported that the bus driver had died and that the youngest of the passengers was 4 years old.

Outside a school in North Macedonia’s capital, Skopje, some children cried after learning the news that five of their peers were killed. “Ergin was my friend. He was a very good boy. Very nice,” 11-year-old Blerim Bushi told Reuters. “I am so sorry that they died.”