Marla Cooper says that her two uncles wanted to return to search for the cash, but her father refused. She believes this was because the FBI search was just beginning to take shape.
After that Thanksgiving Day she never saw her uncle again. She was told he died in 1999.
In 1980, the case was put in the spotlight once again, after a young boy found $5,800 in $20 bills from the ransom money decomposing along the banks of the Columbia River.
Marla Cooper showed ABC News a 1972 Polaroid picture of her uncle, a Korean War veteran. She claims the picture is eerily similar to the composite sketch authorities put out in the 1970s -- and even says that one of the flight attendants that was in the hijacked plane agrees.
"Of all the photos that have been brought to her attention over the years my uncle really looked like him, 'this sure looks like the guy' is what she said," she told ABC News.
ABC News tried to reach some of those flight attendants for comment but was unsuccessful.
According to Marla Cooper, two conversations with her parents initially made her suspicious. The first was in 1995 with her father just before he died.
"My father made a comment about his long lost brother, my uncle L.D. … he said 'don't you remember he hijacked that airplane?'" she said.
At the time she was unable to embrace such an incredible story. But in 2009 it came up again while speaking with her mother.
"A couple years ago my mother made a comment, another comment, a similar comment that she had always suspected that my uncle L.D. was the real D.B. Cooper," she said.
Marla Cooper eventually contacted the FBI, and recently provided them with the guitar strap seen on the photo so that they can check it for fingerprints.
"I contacted the FBI as soon as I was sure that what I was remembering were real memories," she said. "There's a crime that's taken place that hasn't been solved and I'm the only one, as far as I know, who knows what happened."
She also said that her uncle was obsessed with the Canadian comic book hero Dan Cooper, and even had one of the comic books thumb-tacked to the wall. She added that she thinks her uncle didn't expect to survive the hijacking.
"I'm not convinced that he wasn't on somewhat of a suicide mission. I really think he jumped out not expecting to live," she said.
The FBI also obtained a partial DNA sample from the black JCPenney clip-on tie Cooper left on the plane before jumping out. The FBI extracted the sample in 2001.
Geoffrey Gray, the author of the forthcoming "Skyjacking," is the first journalist to look at the FBI files related to the case and said that several people have come forward over the years claiming that their long-lost relative was the hijacker.
"I think that right now we're on the verge of like a new round of Cooper mania...The story of Cooper is really the story of people coming forward claiming that they heard a long-lost uncle say something," Gray said. "We're fascinated with genteel thiefs and here's a guy who committed a crime where there were relatively few victims, dressed in a suit, clip-on tie and made a getaway and was never seen again."