Authorities in Bulgaria have launched an investigation into a passenger bus fire that left at least 45 people dead and seven injured, according to state media.

After initially reporting 46 deaths, authorities now say it is unclear whether 45 or 46 people died in the incident, according to Bulgarian National Television (BNT), CNN reported.

Authorities believe the bus was carrying a total of 52 passengers, 12 of them children, when the fire erupted at around 3 a.m. local time Tuesday (8 p.m. ET Monday).

The bus, which had a North Macedonian registration, was traveling on Struma highway, west of the capital, Sofia. Most of the passengers were tourists from North Macedonia, authorities said. The bus driver was among the dead, according to BNT.

The seven people who survived are in stable condition, according to BNT.

Bulgarian Interior Minister Boyko Rashkov said bodies were “clustered inside and are burnt to ash.”

“The picture is terrifying, terrifying. I have never seen anything like that before,” he told reporters at the site.

The cause of the accident was unclear but the bus appeared to have hit a highway barrier either before or after it caught fire, Bulgarian officials said.

Footage from Nova TV Bulgaria showed the charred remains of the bus on the side of the highway, with the mangled barrier wedged beneath the vehicle.

Emergency responders were seen standing near fire trucks and a police van while an investigator in a white protective suit surveyed the scene taking photographs. Behind the vehicle, damage to the center divider was visible with debris strewn across both sides of the highway.

The coach party was returning to Skopje, the capital of North Macedonia, from a weekend holiday to Istanbul, a trip of about 800 km (500 miles).

“We have an enormous tragedy here,” Bulgarian interim Prime Minister Stefan Yanev told reporters.

North Macedonian Prime Minister Zoran Zaev said he had spoken to one of the survivors who told him the passengers were sleeping when they were woken by the sound of an explosion.

“I am terrified. This is such a huge tragedy,” he told private television channel BTV.

Bulgarian investigative service chief Borislav Sarafov said four buses from a North Macedonian travel agency had entered Bulgaria late on Monday from Turkey.

“Human mistake by the driver or a technical malfunction are the two initial versions for the accident,” he said.

Officials have launched an investigation into the incident, state media reported.