Three passengers have been treated for smoke inhalation after a Greyhound bus en route to Dallas from Los Angeles caught fire on a freeway northwest of Tucson.

Northwest Fire Department paramedics say the driver and 37 other passengers aboard the bus escaped injury. They say the three passengers were treated at the scene for minor smoke inhalation and released.

Investigators have determined that the fire was accidental and resulted from a mechanical problem within the engine compartment.

Greyhound officials say another bus was dispatched to the scene to pick up the passengers for the rest of their trip to Dallas. The bus company also is assisting the passengers with damaged luggage from the fire.

The bus fire was near the scene of a fatal accident earlier Thursday that closed a portion of Interstate 10 for hours.