NEW : Driver fatigue , poor road conditions may have been factors . The bus had left the city of Lima about two hours earlier . CNN 's Mariano Castillo and Esprit Smith contributed to this report . Injuries range from severe head and back injuries to minor scrapes . The tour bus plunged into a ravine in a mountainous area near Cuzco .
NEW : Driver fatigue , poor road conditions may have been factors . The bus had left the city of Lima about two hours earlier . Injuries range from severe head and back injuries to minor scrapes . The tour bus plunged into a ravine in a mountainous area near Cuzco .