On June 6th 1971 at approximately 18:11,Hughes Airwest flight 706 McDonnell Douglas DC-9-31 passenger aircraft collided midair over the San Gabriel Mountains near Duarte,California with a United States Marine Corps F-4B Phantom II.All 44 passengers,the 5-man crew of the commercial aircraft,and the U.S. Marine Corps pilot were killed.What caused this accident to happen,and was there any way to prevent it?