The Pima County Sheriff's Department is now investigating a house fire that damaged three homes on the Northwest side as arson. The fire started in the back of one house and spread to the two on either side of it. The Sheriff's Department said they are investigating this as arson because no one was at the burning house when deputies and fire crews arrived. The call came in around 10 p.m. Thursday night to a neighborhood near Shannon Rd. and Cortaro Farms Road. Neighbors told KGUN9 the flames were extremely high and could be seen more than a block away. Crews were able to knock down the fire quickly. No one was injured. Neighbors on the street had to be evacuated for more than an hour and a half while firefighters knocked out the flames.