The Federal Aviation Administration said a twin-engine Beechcraft Duchess with a flight instructor and student aboard collided with a Mooney MO20 with one person aboard Tuesday afternoon a short distance from McClellan-Palomar Airport about 30 miles north of San Diego.
The planes fell to earth about 1/2 mile apart near a large construction site.
"We hear planes going over all the time, but this one caught our ear and we looked up and saw him coming straight down," construction worker John Peterson told San Diego television station KGTV. "We ran over, but there was nothing we could do."
Another witness told reporters it appeared the two pilots tried to take evasive action, but both turned in the same direction and collided.
Investigators in northern San Diego County have not released the names of the victims and are still trying to determine what caused the collision.