SOFIA, Bulgaria (AP) — Bulgaria and North Macedonia began periods of national mourning on Wednesday, a day after a bus traveling through Bulgaria carrying tourists to neighboring North Macedonia crashed and caught fire, killing 45 people.

The bus with 52 people on board was one in a convoy of buses returning from a trip to Turkey. Though the exact cause of the crash has not yet been determined, the bus apparently ripped through a guardrail on a highway, quickly engulfed in flames and burned out completely. A dozen children were among the dead.

Bulgaria was holding a day of national mourning Wednesday while North Macedonia began a three-day period. National flags of all state institutions were being flown at half mast while entertainment events were cancelled.

Pope Francis also sent a telegram of condolences conveying his prayers, especially for the families of the youngest victims.

“Mindful in particular of the children who have died, he commends the souls of the deceased to the mercy of the Almighty, and assures the grieving families and those who have been injured of his prayers," said the telegram, signed by the Vatican secretary of state, Cardinal Pietro Parolin.

Seven survivors were hospitalized at the main emergency hospital in Sofia, the Bulgarian capital, with severe burns and body injuries.

North Macedonia’s Foreign Minister Bujar Osmani visited the hospital Wednesday, telling reporters he was there to to meet patients and facilitate communication between family members and the institutions in Bulgaria.

Forensic doctors from Skopje, the capital of North Macedonia, were also expected to join their Bulgarian colleagues in Sofia to carry out DNA tests to identify the bodies of those killed in the crash.

Bulgarian Interior Minister Boyko Rashkov told reporters at the crash site Tuesday that he had never "seen something more horrifying” and that the identification process would take time. “The people who were on the bus are turned to charcoal,” Rashkov said.

Among the survivors were five North Macedonia citizens, one Serb and one Belgian, according to North Macedonia’s Foreign Ministry.

Albanian Foreign Minister Olta Xhacka said almost all of the dead were ethnic Albanians, but it was not clear if they were also citizens of North Macedonia.

The burned-out shell of the bus was towed away on a truck on Wednesday, escorted by police on the outskirts of Sofia.

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Nicole Winfield in Rome contributed.