NASA, Boeing and Lockheed engineers determined that the probable cause for the accident was in-flight separation of the right wing while cruising at due to flutter caused by unexplained reduced stiffness of the engine mounts. This was subsequently defined as "whirl mode." Six months earlier, a Braniff International Airways L-188 Electra, Flight 542, disintegrated over Buffalo, Texas at , killing all on board. This second similar crash moved the Federal Aviation Administration to immediately issue a reduced cruise speed directive while investigators tried to determine the cause of the fatal crashes.