Investigators do not yet know if the deceased were plane passengers or if they were workers at the building where it crashed, a police officer told local station KTLA.
Television footage showed a gaping hole in the roof of the building, with smoke pouring out.
“All we hear is like a loud noise, boom, and that’s it. Then we started running out” of the building, Jerome Cruz, one of the workers who witnessed the scene, told CBS News.
The plane was a single-engine Van’s RV-10, a small model with four seats, according to the Federal Aviation Administration, which has opened an investigation.
Another crash near Fullerton Airport injured two people in November.