Investigation Summary
The aircraft, a Douglas DC-6B named 'Mainliner Denver', was operating a multi-leg transcontinental flight. After a stop in Denver, it departed for Portland at 18:52 MST. At 19:03, witnesses on the ground and controllers at Stapleton Airport observed two bright flashes in the sky. The aircraft had been shattered by a massive explosion in the number 4 baggage compartment, located in the aft fuselage. The tail section separated immediately, and the remaining structure disintegrated as it fell. Investigators from the Civil Aeronautics Board (CAB) and the FBI utilized a warehouse to perform a full-scale 'mock-up' reconstruction of the fuselage, a pioneering technique at the time. They found soot deposits and metal fragments contaminated with dynamite byproducts. Suspicion fell on John Gilbert Graham after agents discovered he had purchased multiple life insurance policies for his mother, Daisie King, just before the flight. A search of Graham's home uncovered bomb-making materials identical to those found in the wreckage.
Final Conclusions
The probable cause was the detonation of a dynamite bomb in the number 4 baggage compartment. The device consisted of 25 sticks of dynamite, two blasting caps, and a timer set for 90 minutes. The motive was identified as insurance fraud and personal matricide. John Gilbert Graham was convicted of first-degree murder and executed in 1957. At the time of the crash, no federal law existed specifically prohibiting the bombing of an aircraft; Graham was prosecuted for the murder of his mother to ensure a capital conviction.
Photographic Evidence (1)
Video Analysis
The night a son blew up his mother's flight over Colorado — On Nov. 1, 1955, United Flight 629 took off from Stapleton Airport in Denver. Eleven minutes later, the plane exploded over the Colorado countryside.
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Airframe & Maintenance
Crew Experience
Captain
Age 41
First Officer
Age 26
Flight Engineer
Age 38
Systems & Failure Modes
Number 4 Baggage Compartment
The aft cargo hold where the dynamite bomb was placed; the explosion here caused the tail section to separate.
Fuselage Station 718
The specific structural point identified by investigators as the epicenter of the blast.
Interesting Facts
- 01The explosion was caused by 25 sticks of dynamite connected to a timer and a six-volt battery.
- 02This was the first time aircraft wreckage was reassembled in a hangar to solve a bombing case.
- 03The perpetrator, John Gilbert Graham, was only charged with one count of murder (his mother) because no federal law existed for aircraft bombing.
- 04The aircraft had accumulated 11,949 flight hours prior to the accident.
- 05The debris field covered approximately six square miles near Longmont, Colorado.
- 06Investigators initially interviewed approximately 200 occupants across a 140-square-mile area surrounding the crash site to gather witness statements.
Safety Actions & Advisories
Crimes Against Aviation Act
President Dwight D. Eisenhower signed a bill on July 14, 1956, making the intentional bombing of a commercial airline a federal crime.
Known Controversies & Unanswered Questions
Labor Dispute Sabotage Theory
Reference ↗Early in the investigation, authorities considered the possibility that the bombing was an act of sabotage related to a heated dispute between United Airlines management and a local airline union. This theory was investigated but eventually excluded as evidence pointed toward John Gilbert Graham.