Two people were killed from each plane, and the fifth was inside a Chevy dealership that was hit by wreckage, said Wayne Pollack of the National Transportation Safety Board.
The small Cessnas collided Sunday afternoon near the small Corona Municipal airport and a freeway in Riverside County, about 45 miles southeast of Los Angeles, Federal Aviation Administration spokesman Allen Kenitzer said.
Kenitzer did not immediately know where either plane was headed or whether there were any distress calls.
The Corona airport does not have a staffed control tower, he said.
Television pictures showed that the smashed fuselage of one of the planes landed atop a parked car.
Virgil Rodriguez, a manager at the nearby Corona Nissan, told local broadcasters that sales staff and customers were outside when they heard a crash and saw debris come raining down.
"It was like an explosion in the air and we seen debris flying all over the place," he told CBS station KCBS-TV.
"We all ran inside, everybody just ran for cover because everything was flying everywhere. I believe a body landed on top of a car, one in the middle of the street and the plane landed in the back of our building and crushed a couple of cars."
"At first we thought it was only one (plane), but we after everything came down, there was one in the back of our building and one in the middle of the street," Rodriguez said, adding that the planes appeared to be Cessnas.
Rodriguez told KCBS bodies landed all over the dealership. "We got four bodies on our property - two in the plane, one in the driveway and one on the car."