Fire crews were battling a large fire Thursday evening that was blazing in the hills of San Bruno, south of San Francisco.



It was unclear what sparked the large fire, which was preceded by a large explosion. Firefighters were pouring massive streams of water on homes that were fully engulfed in flames.



A spokeswoman for the San Bruno Police Department said authorities were trying to determine what caused the fire, a few miles northwest of San Francisco International Airport.



Video footage from the scene showed thick clouds of dark smoke billowing across the hills and orange balls of flame.



-- Robert J. Lopez