Authorities believe the bus was carrying a total of 52 passengers including 12 children, most of whom were tourists from North Macedonia, when the incident occurred at around 3 a.m. local time Tuesday.
The bus, which had a North Macedonian registration, was traveling on the Struma highway west of the Bulgarian capital Sofia. The bus driver was among the dead, according to Bulgarian National Television (BNT).
After initially reporting 46 deaths, authorities later said it was unclear whether 45 or 46 people had died, BNT reported.
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).
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.
“This is a huge tragedy,” North Macedonian Prime Minister Zoran Zaev told reporters in Sofia and expressed his condolences to relatives of the victims.
Zaev said the passengers were all from North Macedonia but appeared to include a Serbian citizen and a Belgian citizen. It was unclear whether the two were among the victims or injured.
Zaev said he had spoken to one of the seven survivors who told him the passengers were sleeping when they were woken by the sound of an explosion.
He said people sitting at the back of the bus were able to break a window and jump out.
Zaev said the passengers were from various communities in North Macedonia, a country of 2 million that borders Bulgaria and is home to an ethnic Albanian minority.
In Skopje, ethnic Albanian Osman, 31, told Reuters he had come to the office of the travel agency with his brother and sister seeking information about their parents.
“We do not know if they were on the bus that crashed or not. We have no information about them. The agency is not answering the phone. Perhaps we will need to go to Bulgaria,” he said.
Albanian Foreign Minister Olta Xhacka said the passengers were from North Macedonia’s ethnic Albanian community.
“Great grief for the 45 lost lives of Albanians from Northern Macedonia during the tragic accident in Bulgaria,” he said on Twitter.
Bulgarian officials have launched an investigation into the incident, state media reported.