The windows shook as dynamite aboard an airplane exploded over Conrad Hopp’s family farm in northern Colorado 70 years ago.

Hopp, then 18 years old, saw a ball of fire streaking across the night sky and rushed with his brother toward where the burning wreckage came down, dodging objects that turned out to be the bodies of victims of the first confirmed case of sabotage against a commercial U.S. airliner.

A memorial to those who died is set to be dedicated Saturday, the 70th anniversary of the bombing, which killed all 44 people on board.

Until now, the fate of the victims has been overshadowed by the dramatic details of the bombing, the glaring absence of a federal law against attacking a plane and the meticulous investigation into what happened.

The United Airlines flight took off a few minutes late after a layover in Denver on its way to Portland. Oregon. Most of the passengers were from somewhere else, said Michael Hesse, the president of the Denver Police Museum who spearheaded the effort to create a memorial at the air traffic control tower of the city’s former airport, which is now part of a brew pub.

That’s part of the reason no memorial was ever built before, Hesse suggested. The granite slab with victims’ names listed within the outline of a plane will also include the seals of local and federal law enforcement agencies who responded to the bombing.

A separate memorial at the crash site, where homes are now being built, is also in the works.

The blast, a wake up call to the danger posed to the emerging airline industry, wasn’t terrorism but the result of a personal grudge. John Gilbert Graham confessed to putting 25 sticks of dynamite attached to a timer into the luggage of his mother, who had put him in an orphanage as a boy. He bought a travel life insurance policy in her name, apparently at a vending machine at the airport.

Graham planned to cover his tracks by having the plane explode over the mountains in Wyoming, making it difficult to investigate the crash. But the flight’s delay caused the plane to explode over beet fields near Longmont, Colorado, allowing investigators to piece together the wreckage and interview eyewitnesses.

At the time, federal law outlawed attacks on trains and ships but not airplanes, leading Graham to be swiftly prosecuted in state court for a single count of pre-meditated murder for killing his mother, Daisie King. None of the others who died were named as victims.

Congress outlawed attacks on airplanes shortly after Graham was convicted. Graham, who was married with two young children, was executed in January 1957.

FBI records show Graham may not have been the first saboteur of an airliner: Explosives were strongly suspected in a 1933 crash of a United airliner over Indiana that killed seven people, but experts at the time also said it could have been caused by a gas vapor explosion.

The FBI said its probe of the Colorado crash provided a template to guide future complex airline investigations, including the terrorist bombing of a Pan Am jumbo jet over Lockerbie, Scotland, in 1988. That attack, using a bomb hidden in a cassette recorder packed inside a checked suitcase, led to the strengthening of baggage screening procedures.

After the Colorado bombing, Hopp joined his family and his girlfriend — who would later become his wife — to help find and protect bodies from looters until others could take them to a makeshift morgue. Hopp’s father later broke down recounting what had happened and the family developed an unspoken agreement not to discuss the bombing. For years, Hopp said he woke up after having bad dreams about bodies.

AP Video by Thomas Peipert