The discovery was made as investigators examined the crash site and firefighters stood by to make sure the fire did not rekindle in advance of predicted windy conditions, said Los Angeles County Fire Department Inspector Tony Imbrenda.
The plane went down Monday on a ridge in the mountain range north of Malibu near Calabasas, about 25 miles west of downtown Los Angeles.
The crash of the single-engine Cessna 172 sparked a half-acre brush fire that was quickly controlled.
The plane, which had taken off from Santa Monica airport, was nearly completely destroyed. The debris field was very compact, said Howard Plagens, lead investigator for the National Transportation Safety Board.
"It went in at a pretty steep angle, it looks like," he said.
Three people on the other plane, also a Cessna, suffered minor injuries as it landed wheels-up on a fairway at Westlake Golf Course, about six miles away. One was hospitalized after complaining of back pain. That person and the other survivors were being interviewed by Federal Aviation Administration investigators Tuesday.
Stunned golfers said the single-engine plane hit a tree, spun around 180 degrees and came down surprisingly gently on the grass.
"All we heard was a thud and then he made a gentle bounce and slid down the center of the fairway," golfer Aaron Jesse, 47, told the Los Angeles Times.
Radar records showed the two flight paths crossed at 2:01 p.m., according to Plagens.
The plane that ended up on the golf course was flying west at an altitude of 3,500 feet as the other Cessna headed east, said FAA spokesman Alan Kenitzer.
Investigators will determine each pilot's field of view, and whether they might have been able to see each other when they collided, Plagens said.
Both planes were flying in an area without air traffic control services.
The NTSB and FAA were investigating. A preliminary report was expected within five days.
FAA records showed the plane on the golf course was manufactured in 1980 and is registered to Ameriflyers of Florida, LLC. A message left at a number listed for the company was not immediately returned.
Firefighters were expected to remain on the scene near Calabasas for two more days. The National Weather Service predicted winds up to 45 mph in the area on Wednesday
.