Fresno firefighters say the 11-unit building on Vassar Avenue near Blackstone Avenue caught fire shortly before 11 p.m. Friday.
Half of the units in the building suffered fire and water damage. The fire started in an upstairs unit -- and quickly spread through the attic. Firefighters say neighbors began helping each other out of their homes -- as soon as they saw the huge flames.
Investigators say they have not been able to talk with the man who lives in the apartment where the fire started.
Fresno Fire Department Battalion Chief Casey Clark said, "It was quite a lot of fire to begin with. People were rushing around collecting their children. Fortunately, in this case everyone got out rapidly. Neighbors were helping neighbors, which we love to hear."
Jason Knox of Fresno said, "All of the sudden people just started running, screaming 'fire fire.' The fire went voosh... It just got out of hand. It just went up so quickly."
The fire displaced 21 people. The Red Cross is helping them find a place to stay. Neighbors say there were no fire extinguishers in the emergency cases along the outside of the building.
Investigators still don't know what caused the fire.