A passenger bus caught fire and exploded in southern Russia Thursday, killing at least five people and wounding 12, emergency and police officials said. Investigators said terrorism was the likely cause.

The incident took place in Russia’s troubled North Caucasus, which is plagued by violence from feuding criminal groups, remnants of Chechen separatist groups, and other militant groups that target government and police. Authorities did not immediately name any suspects.

The RIA-Novosti news agency, citing an unnamed law enforcement official, said the blast may have been caused by an explosive device in the road.

However, a duty officer for the North Ossetia Interior Ministry told The Associated Press that the device had been planted on the bus, possibly while it was parked near a police post awaiting a sweep by officers.