National police spokesman Donald Gonzalez says investigators have not ruled out mechanical causes. But he acknowledges that some witnesses say the bus was attacked.

Firefighter Carlos Hernandez told a local radio station Monday that someone tossed a fire bomb at the bus in the capital, Guatemala City.

Street gangs have demanded protection money from bus drivers on the route in the past and killed drivers who refused to pay. Rights activists say 60 drivers were killed in 2010.