An amateur-built plane that slammed into a furniture warehouse near Fullerton Airport, killing the pilot and his teenage daughter and injuring 19 victims on the ground, had defects in a door that appears to have contributed to the collision.

A witness saw the left main cabin door open during flight, and an examination of the door showed the interior handle was not in the locked position.

The door to a pilot's plane appeared to be ajar as it took off from Fullerton Airport, moments before it crashed into a warehouse, according to investigators.

Authorities believe that plane door may have been open before the small jet ended up crashing into a Fullerton warehouse, killing the pilot and his daughter.