OAKLAND - Investigators say a fire at a warehouse in Oakland that claimed 36 lives progressed rapidly, trapping people on second floor.

Special agent in charge of the San Francisco office of the Bureau of Alcohol, Tobacco, Firearms and Explosives Jill Snyder said Wednesday people on the second floor of the warehouse did not know there was a fire on the first floor until it was well developed.

She says smoke went up the stairwells to the second floor.

The fire broke out at the warehouse during a dance party Friday night.