( CNN ) -- At least 40 people died and 20 were injured early Thursday morning when a bus plunged into a ravine in a mountainous region near Cuzco , Peru , authorities said . The bus crashed about 5:30 a.m. near the town of Espinar , Peru , Lt. Edilberto Tunquipa of the Espinar fire department said . Interviews with survivors pointed to driver fatigue as one of the possible causes of the crash , Tunquipa said . The bus had left the city of Arequipa about seven hours earlier , heading to the town of Santo Tomas in the mountains , he said . The poor conditions of the road were another possible factor , he added . No international tourists were believed to be on the bus , though authorities could not confirm that , Tunquipa said . The 20 people injured were in hospitals , their injuries ranging from severe head and back injuries to minor scrapes , he said . Most of the victims were believed to be locals who work in Arequipa and were returning home to their small towns for Christmas , he said . The exact number of people on the bus remained unknown . The bus holds 52 people , Tunquipa said , but it was overcrowded , with people standing in the aisle . CNN 's Mariano Castillo and Esprit Smith contributed to this report .
NEW : Driver fatigue , poor road conditions may have been factors . The bus had left the city of Arequipa about seven hours earlier . Injuries range from severe head and back injuries to minor scrapes . The tour bus plunged into a ravine in a mountainous region near Cuzco .