Investigators with the National Transportation Safety Board and La County Sheriff's Department continued combing through the plane wreckage Tuesday.
According to Allen Kenitzer with the Federal Aviation Administration, the first plane was heading west at 3,500 feet, and the second had just left Santa Monica Airport around 2 p.m. Monday for an engine test flight and was heading east at 3,100 feet.
For reasons unknown at this time, FAA radar data showed the flights merged about 8 miles east northeast of Ventura. The pilot of the first plane managed to maneuver a belly-flop landing on the Westlake Golf Course. The pilot and two others on board sustained minor injuries.
The collision sent the other plane crashing into the Santa Monica Mountains near Calabasas. Preliminary investigation shows that the aircraft was a single-engine four-seat Cessna, and it appears to have gone into a complete dive into the side of a hillside and burst into flames. Investigators believe they have found all the victims in the wreckage.
"They were in a spot where they were not receiving any air traffic control services," said NTSB official Howard Plegens.
Officials say 10 minutes before the collision, both pilots had been in routine contact with the Santa Monica control tower, and there were no problems. Officials are investigating how the planes ended up on a collision course.
The FAA and the NTSB say they will be investigating both crash sites. Federal officials say they hope to have a preliminary report on the cause in about a week or two.