LONGMONT, Colo. — Conrad Hopp can talk about supper that night, how he and his family sat down for a meal in the house where he grew up, on a farm east of Longmont. It was November 1, 1955 , and Hopp was 18, less than a year out of school. The family had spent all day in the fields, like so many days before, harvesting sugar beets.

"And then we hear this loud explosion that shook all the windows in the house," Hopp said. "We looked outside, and we could hear the roar of the engines — that's how you knew it was a plane — and the ball of the fire coming through the air."

Hopp and his brother ran outside and lost sight of the flaming wreckage behind the outbuildings of the farm. They jumped in Conrad's car, a '54 Chevy, and drove across the rows of alfalfa, dodging debris that had fallen from the sky. They reached an irrigation ditch and a thicket of trees, and Conrad parked the car, his headlights shining on the back of an airline seat.

His brother climbed over a fence and ran to the wreckage.

"Go get some coats," he yelled to Conrad, as a chilly night set in.

Here in his memory is where Hopp stops himself.

"I'm sorry," he says. "I can't do this."

When he turned back to his car, he saw the front of the airline seat and a body still strapped in by the seatbelt.

The night a Denver man blew up his mother's flight

A "ball of fire that lit up the eastern skies"

The explosion of United Air Lines Flight 629, 64 years ago this week, was one of the first attacks on a commercial airliner in the United States. It was also the deadliest act of mass murder in Colorado history, killing all 44 people on board – a five-person crew and 39 passengers, including a 13-month-old boy.

The DC-6B aircraft took off from Denver's Stapleton Airport on Nov. 1, 1955, at 6:52 p.m., bound for Portland, Ore. Eleven minutes later, according to the accident report from the Civil Aeronautics Board, an employee in the Stapleton control tower reported seeing a bright flash of white and a flare in the distant northern sky.

The air traffic controllers touched base with every plane in the Stapleton airspace. Flight 629 was the only one not to respond.

The explosion, investigators soon learned, was the culmination of a young man's anger toward his mother, who was on the flight, and the 25 sticks of dynamite he packed in her suitcase. The plane blew up at about 5,780 feet above the ground, over farmland eight miles east of Longmont. The wreckage was strewn across six square miles, near where Interstate 25 now meets Colorado 66.

In the moments after the explosion, hundreds of callers flooded the Longmont Police Department, the Rocky Mountain News reported, and thousands began to flock to the area, curious about "the ball of fire that lit up the eastern skies for miles."

Keith Cunningham, the Longmont police chief at the time, called the Colorado State Patrol and sent every police officer and firefighter in the city to the scene, and dispatched every ambulance, too. A few minutes later, the newspaper reported, a patrolman radioed back: "No ambulances are necessary."

Denver District Attorney's Office The 44 victims of the United Air Lines Flight 629.

The investigation begins

About two miles to the south, Martha Hopp, then Conrad's girlfriend and a senior at Mead High School, was also sitting down for supper when the explosion happened. Martha and her father, like just about everyone else across the county, ran outside and drove toward the wreckage.

"And when we looked around, every road was lights," Martha said. "Up on the hills, everywhere you looked was lights because everyone was doing the same thing – to go see what happened."

About a quarter-mile up the road, they began to see silverware from the plane, littered across the ground — then, letters, pieces of paper, and large dinner trays. Martha joined Conrad in his father's two-ton truck, and they spent the night looking for bodies.

One victim fell into a straw pile, and Conrad helped fork the pile apart to find the body. Using the truck, Martha marked where the bodies were found by driving circles around them.

FBI.gov The farmlands where Flight 629 east of Longmont. The plane's tail can be seen in the bottom right of the photo.

"At that point and time, you don't even realize what you're experiencing at the time until after the fact," Conrad said. "When you try to think about it later, it bothers you. It takes its toll."

The Rocky Mountain News described the night as a "scene of death and horror under flickering flames," and the searchers in a state of shocked daze.

"Hundreds of persons milled around in the beet field between Longmont and Platteville," reporter Jack Gaskie wrote for the next day's newspaper. "There were certain things to do – cover the bodies, make futile efforts to quell the flames of the fiercely burning wreckage. But for the most part, they stood around in quiet, stunned groups, waiting."

Martha went to school the next day, and Conrad kept helping. Hundreds of the searchers formed a line, he said, standing about an arm's length apart, and walked across the vast fields, combing the ground for every piece of the wreckage.

It was meticulous work.

The Civil Aeronautics Board report, for example, detailed how the tail of the plane landed some 4,600 feet to the southeast of the motors and the wings, which created deep craters at impact, and the front of the aircraft landed about 600 feet to the north of those craters. Another panel from wing landed 600 feet to the south of the craters.

The rest of the wreckage was scattered across the fields. The investigators likely mapped out where every piece of the plane was found, said Jeff Guzzetti, a former accident investigator with the National Transportation Safety Board and Federal Aviation Administration.

The process became even more meticulous once the wreckage was taken to a warehouse at the Stapleton airport, where investigators began to reconstruct the body of the plane around a frame of chicken wire, piecing together hundreds of scraps of the disintegrated aircraft.

"It's really the only way to fully visualize where on the aircraft the source of the explosion was," Guzzetti said

Within hours of the crash, it had become clear that an explosion of "such great intensity" wasn't a malfunction of the plane, the Civil Aeronautics Board wrote in its report.

Within days, the investigators had pinpointed the source of the explosion: A "dynamite-type" blast in baggage compartment No. 4 in the cargo hold, the report said.

At this stage, six days after the crash, the Civil Aeronautics Board contacted the FBI. The investigation became criminal.

Read the Civil Aeronautics Board's full accident report

FBI.gov The wreckage of the plane in a warehouse at Stapelton Airport, where investigators reconstructed the aircraft.

A mother and son who "fought like cats and dogs"

While a team of investigators combed through the wreckage of the plane, another team began the long task of compiling background information on each of the 44 victims.

Part of this investigation included finding out what luggage each passenger was carrying, and then comparing that information with how much of their bags were destroyed, according to the FBI's account of the explosion. This would narrow the search for the passengers who had the most badly-damaged luggage or luggage coated in foreign residue.

One passenger whose luggage was almost destroyed was that of Daisie E. King, a 54-year-old woman from Denver.

King, according to the FBI, was carrying several items with her on the plane that were recovered by investigators: Personal letters, a personalized checkbook, $1,000 in traveler's checks, an address list, two keys for safe deposit boxes and newspaper clippings about her family, including her 23-year-old son, John "Jack" Gilbert Graham.

Graham, the newspaper clippings revealed, had been charged with forgery several years earlier and was placed on a "most wanted" list by the Denver County District Attorney.

The investigation focused on King and her family, especially the fraught relationship with her son.

Graham, the FBI learned, was set to receive an inheritance but the mother and son had argued for years. He had lived with various family members through the years and left home at 16. Graham later returned to Denver, where his mother had opened a drive-in restaurant and allowed Graham to run it.

But the mother and son still "fought like cats and dogs," according to the FBI, and one witness told investigators that Graham might have been embezzling money from the business.

There were other red flags about Graham.

In September 1955, two months before the plane explosion, an explosion damaged the restaurant. Graham blamed it on a disconnected gas line. That same year, Graham's new 1955 Chevrolet pickup truck stalled on a railroad track and was struck by a train. He blamed it on bad luck.

When asked about his mother's trip on the day of the explosion, Graham offered little for investigators. King was flying to Alaska to visit her daughter – Graham's sister – and he claimed he didn't know what his mother had packed in her luggage, other than shotgun shells and other ammunition for hunting caribou.

Then investigators interviewed Graham's wife, Gloria.

She provided the FBI some background on the couple: Married in 1953; the parents of two young children. They lived in the Lakewood area, on Mississippi Avenue, and had shared the home with Graham's mother for about a year.

Gloria Graham also said she was unsure what King might have packed for her trip. But she offered an interesting detail.

On the day of the crash, Jack Graham was planning to give his mother an early Christmas present, Gloria said, believed to be a set of small tools. He had apparently searched all day for the special gift, a neighbor later told investigators. Graham, his wife recalled to the FBI, brought the package into the house and carried it to the basement, where his mother had been packing her luggage.

Gloria was unsure whether King had received the package, but assumed that she did.

King finished packing, and the family loaded into Graham's 1951 Plymouth and headed across town to the airport.

Read the Rocky Mountain News' coverage of the explosion

Rocky Mountain News/Denver Public Library The Rocky Mountain News after the revelation of Graham's "Christmas gift" for his mother, Daisie King (pictured on left)

The number of victims "made no difference to me"

The day after investigators interviewed Graham and his wife, they called the couple back. They had received a few tattered pieces of luggage, believed to have belonged to Daisie King, and they asked Jack and Gloria to come down to the FBI office in Denver.

The Grahams agreed, and at the office, they identified a bag belonging to King. The agents told Gloria Graham she could leave but asked her husband to stay behind for a few more questions.

With Jack Graham alone, the agents questioned him about the toolset he reportedly bought for his mother.

Why had he made no mention of the gift and his wife did?

And at the airport, why did he purchase a trip insurance policy in his mother's name? Why did he become sick after her plane took off?

The discrepancies, according to the FBI, were enough to consider Graham as a suspect.

Graham offered to take a polygraph test and gave the agents permission to search his property. At Graham's home, the investigators found a small roll of copper wire – similar to the type found on a detonating primer cap – inside the pocket of one of Graham's shirts. They also found the trip insurance policy that Graham had purchased at the airport on the day of the flight, hidden in a bedroom chest.

Graham's story began to unravel. He admitted to causing the explosion at his mother's drive-in restaurant and to leaving his Chevrolet pickup truck on the railroad tracks.

Then he admitted to the explosion of Flight 629. He said he built a time bomb, with 25 sticks of dynamite purchased in Kremmling, two electric primer caps, a timer and a six-volt battery.

In jailhouse conversations with psychiatrists, Graham detailed how he slipped the homemade bomb into his mother's suitcase and fastened the luggage. At the airport, Graham dropped off his wife and children and his mother at the terminal door and drove to a parking lot. He set the timer on the bomb to 90 minutes and took the luggage to the United counter. The suitcase was 37 pounds overweight. Records showed that King paid the $27 fee, according to the Rocky Mountain News, and the luggage was loaded onto the plane.

At the airport, Graham stopped by a vending machine a paid $1.50 for the trip insurance policy of $37,500 in his mother's name, and named himself the beneficiary.

"Later on that evening, after my wife and I had returned home," Graham said, according to the Rocky Mountain News, "we heard over the radio ... that all passengers aboard had been killed."

The psychiatrists, though, were still curious: Why did Graham do it?

He told the doctors that he realized there would be dozens of other people on the plane.

"But the number of people to be killed made no difference to me," he told the doctors. "It could have been a thousand. When their time comes, there is nothing they can do about it."

The FBI investigated the bombing but handed over the case to Denver District Attorney Bert Keating, who charged Graham with murder. Officials explained that a state murder charge was "the more definite" law – at the time, there wasn't a specific federal law for blowing up a commercial airliner – the Rocky Mountain News reported, and Keating moved for a quick trial.

The case went to court in April 1956, five months after the explosion, and the trial was the first in U.S. history to be televised. Graham's attorneys had argued that his confession to FBI agents was made under duress, but a federal judge dismissed their motion, and Graham's confession stood as evidence.

Graham did not testify, and none of the defense's witnesses refuted the prosecutors' evidence.

On May 5, 1956, the jury deliberated for 69 minutes and found Graham guilty, recommending the death penalty.

A judge sentenced Graham to be put to death in August of 1956. The execution was delayed once but later affirmed by the Colorado Supreme Court.

On January 11, 1957, a little more than 14 months after the explosion, Graham was executed in the gas chamber at the Colorado State Penitentiary.

Rocky Mountain News/Denver Public Library John Gilbert Graham in an early court appearance.

'You just kind of want to forget about it'

The Hopp family saw it in the alfalfa.

In the years after the explosion, they'd harvest the fields and find a bare spot in the crop. It was where a body fell into the ground, and the alfalfa didn't grow back.

They'd find small items buried in the dirt; pens and eyeglasses, small personal effects that fell with the bodies. Up the road, the two engines from the plane stayed buried in the ground for several years, Conrad said. When one of their cows died shortly after the explosion, they found a hunk of metal lodged inside of it.

Hopp's father wasn't a superstitious man, he said, but after the explosion, the longtime farmer refused to water the fields at night on the east side of the farm, where the wreckage landed. Hopp's brothers would say they heard ghosts.

Hopp, himself, tries not to think about the explosion often. He tries not to think about it if he doesn't have to.

"It's something you put back in your mind," Hopp said. "You just kind of want to forget about it."

Today, the rolling farmlands look about the same as they did in 1955, and Hopp can picture where everything happened.

He can spot the two trees near where the tail of the plane landed. He can see where he and his brother took off across the farm toward the wreckage, where he saw that first body strapped in the airplane seat.

The land will likely become a subdivision one day, Hopp said. He's seen the neighborhoods gradually grow across the area, as they have everywhere along the Front Range.

And Hopp wonders if the people in those homes will know what landed in their backyards, if they'll know United Air Lines Flight 629 ever happened at all.

Conrad Hopp points to where the wreckage of United Air Lines Flight 629 crashed east of Longmont.

This story was originally published by Ryan Osborne and Robert Garrison at KMGH.