(KRON) — A loud “boom” was heard at a two-story apartment complex prompting fire crews to respond Thursday afternoon, according to the Santa Rosa Fire Department.

SRFD arrived in under four minutes and located a fire in the backyard of an apartment that had spread to both floors of one unit and into the attic. The blaze had broken all sliding glass doors on both floors which made easier access for crews to enter, the fire department said.

Firefighters climbed to the roof to cut ventilation openings while crews inside the apartment as well as in the backyard worked to extinguish the blaze. The fire was contained within 20 minutes of crews arriving, SRFD said.

The fire did not spread to the other two apartments. Crews searched all three apartments and found no occupants.

Photo: Santa Rosa Fire Department

It was later learned a neighbor had knocked on the door of an apartment to alert the occupants of an exterior fire. The occupants safely left the building with their pets before crews arrived.

The Red Cross is assisting the three people from that unit who are now displaced. The occupants from the other two apartments were able to return back to their units.

Damages are estimated to be around $250,000. The cause of the fire is currently under investigation.