Firefighters transported two patients from the scene in moderate condition. The injuries are not believed to be life-threatening.
The crash was reported just before 2 p.m. to the east of Fullerton Municipal Airport, in the area of N. Gilbert Street and Artesia Avenue.
The wreckage of the plane was upright at the scene and partially wrapped around a tree next to a commercial building. While the cockpit appeared seriously damaged, it wasn't clear if any of that was caused by the crash itself or by firefighters cutting open the cockpit to extricate the occupants.
Firefighters say the pilot radioed in an issue with the plane while trying to return to Fullerton airport. The crash site was about a half-mile short of the runway.
FAA records indicate the aircraft is a single-engine Mooney M20F built in 1969.
The cause of the crash remains under investigation.