Now 48, Cooper said she recently recounted for the FBI her recollection of a Thanksgiving holiday in 1971 at her grandmother's home in Sisters, Ore.
"I was 8 years old, so I can't tell you exactly what he said, but I do remember the words: `Our money problems are over. We just need to go back and get the money,"' she said in an interview with The Associated Press on Wednesday.
While federal investigators say solving the hijacking is a low priority because present-day criminals pose a greater threat, the case holds a prominent place in American folklore: the guy who pulled an incredible heist and got away.
"We're desperate to believe in people who can do things we can't," said Geoffrey Gray, who has written a book about the case.
The FBI isn't convinced D.B. Cooper even survived the jump but has chased more than 1,000 leads in the case. The agency said Monday it was following a new lead, but FBI agent Fred Gutt declined Wednesday to say whether Marla Cooper was their source.
"It is an unsolved crime and we are obligated to address that if new, credible information comes to us," Gutt said.
Marla Cooper, whose comments were first reported by ABC News, said she recalled two of her uncles, including an uncle she knew as "L.D.," plotting something "underhanded" during that Thanksgiving visit to her grandmother's house.
"I knew they weren't shooting straight with me when they were teasing me and telling me they were going turkey hunting," she told the AP.
"I was a witness to them returning from their so-called turkey hunt early the next morning ... when my uncle L.D. was very injured and heard them telling my father that they had hijacked an airplane," she said.
Over the years, Marla Cooper said she never gave much thought to the incident until she pieced together her memories with comments made first by her father shortly before his death in 1995, and later by her mother two years ago.
After her mother's comments spurred her memory, Marla Cooper said she looked up the story of D.B. Cooper and "over the next few days, I was just flooded with memories of what happened."
She said she contacted the FBI after she "was certain that what I was remembering were real memories and not imagined." When agents didn't immediately follow up, she spoke with a retired law enforcement agent who later talked to federal investigators.
On Nov. 24, 1971, a man who gave his name as Dan Cooper claimed shortly after takeoff in Portland, Ore., that he had a bomb, leading the flight crew of the Northwest Orient plane to land in Seattle. Passengers were exchanged for parachutes and ransom money.
The flight then took off for Mexico with the suspect and flight crew on board. The hijacker parachuted from the plane after dark as it flew south, apparently over a rugged, wooded region about 100 miles from Marla Cooper's grandmother's home.
The story has captured the imagination of amateur sleuths for decades in part because it has all the elements of a classic tale, including a hero who is perceived as a Robin Hood-type character, said Gray, whose book "Skyjack: The Hunt for D.B. Cooper" comes out this month.
"We all want to believe in heroes, even if they're bad guys," Gray said.
A generic looking sketch released by the FBI shortly after the hijacking only added to the media frenzy, Gray said.
"That sketch became just a blank portrait for people to fill in with their own fears, suspicions and hunches, and this phenomenon emerged," he said.
But without something more than the memories of an 8-year-old girl, Gray said he remains skeptical Lynn Doyle Cooper is actually D.B. Cooper. He said the FBI's case file is littered with names of dozens of people who suspected a relative might be the infamous hijacker.
"It's unclear what separates Uncle L.D. from this lot," he said.
Seattle-based FBI case agent Larry Carr was tasked with reigniting the case five years ago and the agency posted a "D.B. Cooper Redux" on its site in 2007, urging the public to help solve the enduring mystery.
The FBI released photos of a black J.C. Penney tie the hijacker wore and some of the stolen $20 bills found by a young boy in 1980 along the banks of the Columbia River. In the FBI's recounting, it quoted Carr as saying he thought it was likely that Cooper didn't survive the jump.
But Carr still sought the public's help.
"Maybe a hydrologist can use the latest technology to trace the $5,800 in ransom money found in 1980 to where Cooper landed upstream," Carr said. "Or maybe someone just remembers that odd uncle."
The FBI said a new lead came to the bureau after a tipster initially discussed the case with a retired law enforcement officer, who then contacted the agency. Gutt said only after the FBI contacted the tipster directly did the person speak with investigators.
The lead focuses on a suspect who died more than 10 years ago.
Marla Cooper said her uncle died in 1999 but wouldn't say where he lived before his death.
She said her mother recently provided investigators with a guitar strap belonging to her uncle to be tested for fingerprints.
Investigators have tested a guitar strap from the suspect who is the subject of the new lead, Gutt said Wednesday, but found it wasn't suitable for fingerprint analysis. They are now working with family members to identify other items that can be analyzed.
But the FBI doesn't have a timeframe for how long it will take to vet the lead, which is something they've known about for more than a year, he said.