JAKARTA - Police on the Indonesian island of Sulawesi say a bomb explosion has ripped though a crowded commuter bus killing at least four people and injuring 17. The blast occurred Wednesday but news of the bombing did not reach the Indonesian capital until Thursday, the Associated Press in Jakarta reported. The bus was traveling to the city of Poso, capital of Central Sulawesi province -- a region of Indonesia with a history of sectarian bloodshed. The bombing is the bloodiest incident since a ceasefire between Christian and Muslim gangs was signed last December. Police say they do not yet have any indication who was behind the bombing or their motive, but they are focusing investigations on three passengers who got off the bus shortly before the bomb went off.