The Santa Cruz Fire Department extinguished a fire believed to be arson on Saturday morning.According to the fire department, fire crews responded to a report of a water flow fire alarm at 201 River Street just before 7 a.m.When units arrived on scene, they found a large two story commercial building with a fire alarm sounding and a fire burning on the first floor, spreading to nearby buildings and extending to the second floor.Fire crews made their way into the office, searching for occupants. The fire department says the fire extended into a larger room above several sprinkler heads which allowed the fire to extend across the office.Additional crews set up hose lines inside the building to extinguish the fire. The fire was controlled within 20 minutes and fully extinguished 45 minutes after the time the alarm was received.The Santa Cruz Fire Department Investigator determined the cause of the fire to be arson. Damage to the building was estimated between $750,000 and $1,000,000. The Santa Cruz Police Department and the Santa Cruz Fire Department are currently in the process of reviewing video to identify anyone responsible for setting the fire.One firefighter suffered minor injuries during the fire.