Austin fire investigators believe a cigarette may have sparked a fire at a North Austin apartment complex overnight.

It happened around 2:30 Monday morning on Hearthside Drive, near Burnet Rd. and Research Boulevard.

When crews arrived, there were flames shooting from the first and second floors of the building.

Investigators say a smoke detector inside the apartment where the fire started was working -- it went off, waking the person inside who alerted neighbors.

Thirteen people from four apartments were burned out of their homes.

Property managers were at the complex for hours Monday morning to see the damage for themselves, and talk to residents. Friends also stopped by to help residents gather clothes and other belongings from the smoke and soot filled apartments.

The Red Cross is helping the families who no longer have a place to call home.

Firefighters say the investigation is ongoing.