United Air Lines Flight 629

United Air Lines Flight 629, registration N37559 and dubbed Mainliner Denver, was a Douglas DC-6B aircraft that was blown up on November 1, 1955, by a dynamite bomb placed in the checked luggage. The explosion occurred over Weld County, Colorado, United States, 8 mi east of Longmont, at 7:03 p.m. local time, while the airplane was en route from Denver to Portland, Oregon, and Seattle, Washington. All 39 passengers and 5 crew members on board were killed in the explosion and crash.

Investigators determined that John Gilbert Graham was responsible for bombing the airplane in a bid to kill his mother as revenge for his childhood and to obtain a large life insurance payout. Within fifteen months of the explosion, Graham—who already had an extensive criminal record—was tried, convicted, and executed for the crime. It is the deadliest act of mass murder in Colorado’s history.

Flight and explosion

United Airlines Flight 629 had originated at New York City's La Guardia Airport on November 1, 1955, and made a scheduled stop in Chicago before continuing to Denver's Stapleton Airfield, landing eleven minutes late at 6:11 p.m. At Denver the aircraft was refueled with 3400 USgal of aviation gasoline and had a crew replacement. Captain Hall assumed command of the flight for the segments to Portland and Seattle.

Flight 629 took off at 6:52 p.m. and at 6:56 p.m. made its last transmission, stating it was passing the Denver omni. Seven minutes later, Stapleton air traffic controllers saw two bright lights suddenly appear in the sky north-northwest of the airport. Both lights were observed for 30–45 seconds, and both fell to the ground at roughly the same speed. The controllers then saw a very bright flash originating at or near the ground, intense enough to illuminate the base of the clouds 10000 ft above the source of the flash. Upon observing the mysterious lights, the controllers quickly determined there were no aircraft in distress and contacted all aircraft flying in the area; all flights were quickly accounted for except for Flight 629.

Numerous telephone calls soon began coming in from farmers and other residents near Longmont, who reported loud explosions and fiery debris falling from the night sky—the remains of Flight 629. Ground searchers who reached the crash site determined that all 44 people aboard the DC-6B had been killed. The debris from the accident was scattered across 6 sqmi of Weld County.

Extensive in-air breakup of the entire aircraft had occurred, and major portions of the wings, engines, and center sections were found in two craters 150 ft apart. The large load of fuel ignited on impact, according to fire patterns. The fires were so intense that despite efforts to extinguish them they continued to burn for three days.

There was early speculation that something other than a mechanical problem or pilot error was responsible for the crash, given the magnitude of the in-air explosion. The November 2 edition of The New York Times reported a witness to the tragedy describing what he heard:

"Conrad Hopp, a farmer who lives near the crash scene, said he and members of his family 'heard a big explosion—it sounded like a big bomb went off and I ran out and I saw a big fire right over the cattle corral. I hollered back to my wife that she'd better call the fire department and ambulance because a plane was going to crash. Then I turned around and it blew up in the air.