On 13 May 1975, US Seventh Air Force commander Lieutenant General John J. Burns and his staff developed a contingency plan to retake the SS "Mayaguez" using an assault force composed of men of the Nakhon Phanom 56th Security Police Squadron. Seventy-five volunteers from the 56th would be dropped onto the containers on the decks of the "Mayaguez" on the morning of 14 May. In preparation for this assault five HH-53s and seven CH-53s were ordered to proceed to U-Tapao Royal Thai Navy Airfield for staging. At approximately 21:30, one of the 21st SOS CH-53s (AF Ser. No. 68-10933, call sign Knife 13) crashed, killing 18 security police and the five man flight crew.