The Civil Aeronautics Board report into a mid-air collision near Cheyenne, Wyoming on 15 December 1959.
Civil Aeronautics Board, US Department of Commerce
1960-08-22
US Department of Transportation Special Collection <a rel="nofollow" class="external autonumber" href="http://specialcollection.dotlibrary.dot.gov/Document?db=DOT-AIRPLANEACCIDENTS&amp;query=(select+681)">[1]</a>