SOFIA, BULGARIA—Human error is believed to be the most likely cause of the fiery bus crash in Bulgaria that killed 44 people, including 12 children, early Tuesday, a Bulgarian official said Thursday. The bus, which officials said had 52 people on board, mostly nationals of North Macedonia, was one of four returning from a trip to Istanbul. Turkey. It hit a highway guardrail in western Bulgaria, caught fire, and was reduced to a charred shell. Officials said the passengers were trapped as both exits were blocked. An investigation is underway into the cause of the accident, which has sent a shockwave of grief across the neighboring Balkan countries. On Thursday, Borislav Sarafov, deputy chief prosecutor and head of Bulgaria’s national investigation service, ruled out a terrorist act, adding that investigators also established there had been no blast. Speaking at a press conference in Sofia, Bulgaria’s capital, Sarafov said that for …