An investigation continued Tuesday into an apparent arson attack on a Guatemalan passenger bus that killed at least four people and wounded 18 others, authorities said.

National police spokesman Donald Gonzalez said investigators have not ruled out mechanical causes for Monday's fire but acknowledged that some witnesses said the bus was attacked.

Eyewitnesses told authorities that a woman got on the bus and left a suitcase that exploded and set the vehicle ablaze soon after she got off.

Firefighter Carlos Hernandez told a local radio station Monday that someone tossed a firebomb at the bus in Guatelama's capital city.

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