Marla Cooper told ABC News that she has provided the FBI with a guitar strap and a Christmas photo of a man pictured with the same strap who she says is her uncle, Lynn Doyle Cooper.
After clarifying her childhood memories surrounding the incident and more recent conversations with her parents, she is now sure that her uncle is in fact the notorious man who hijacked and threatened to blow up a commercial plane flying to Seattle in 1971, then parachuted to the ground with $200,000 in hand.
"I'm certain he was my uncle, Lynn Doyle Cooper. Who we called L.D. Cooper," she told ABC News.
Marla Cooper is working on a book about her belief that her uncle is the hijacker, but that is not her main motivation for coming forward.
She said that L.D. Cooper was a Korean war veteran, but he was not a paratrooper. She thinks he lived in the northwest, had children and died in 1999. She said he remained isolated from his family.
He worked with leather and made the guitar strap that she has turned over to the FBI.
The FBI is now searching the guitar strap they received for fingerprints at their forensic lab in Quantico, Va., which will be checked against partial fingerprints obtained from the hijacking. Meanwhile they are hunting for evidence to prove L.D. Cooper was on that plane in 1971.
The real identity of D.B. Cooper has been a mystery since November 24, 1971, when a man calling himself Dan Cooper hijacked a Northwest Orient Airlines plane bound for Seattle from Portland. He ordered the plane to land and demanded a $200,000 ransom and a parachute.
After he received the money in $20 bills and the parachute, he ordered the plane to take off for Mexico. Cooper then did the unthinkable when he lowered the back stairs and jumped out of a speeding 727, thousands of feet over the Pacific Northwest during a raging storm.
He disappeared, despite a massive manhunt that has become the stuff of legend and even a 1981 movie. Throughout the years many leads in the case have became dead ends, and it remains the only unsolved hijacking in U.S. history.
The case was reignited when a male suspect's name was given to the FBI by a law enforcement agent, as was a guitar strap. Sources familiar with the case confirm that it was Marla Cooper who prompted the latest flurry of investigation.
So far no fingerprints have been found on the guitar strap, and the F.B.I. will not officially comment on the case.
"It's a very unique case...Agents have been actively assigned to it and it's passed on from generation to generation...of agents that have worked leads as they have developed," said Brad Garrett, an ABC News consultant and former FBI profiler.
Garrett said the FBI is likely examining the life of Marla Cooper's uncle for more clues that he could be the infamous D.B. Cooper.
"Does this guy's background actually fit someone that could have pulled this off because this guy did have a proficiency in a 727 plane, how low it would fly, how slow it would fly and that you could jump out the back of it," Garrett said.
Family Secrets Could Crack Mystery
Marla Cooper says that as an 8-year-old she recalled her two uncles planning something suspicious at her grandmother's house in Sisters, Oregon -- not far from where D.B. Cooper jumped from a plane with $200,000 in cash one day later.
"My two uncles, who I only saw at holiday time, were planning something very mischievous. I was watching them using some very expensive walkie-talkies that they had purchased," she said. "They left to supposedly go turkey hunting, and Thanksgiving morning I was waiting for them to return."
A day later, Northwest Orient flight 305 was hijacked, and her uncle L.D. Cooper came home claiming to have been in a car accident.
"My uncle L.D. was wearing a white t-shirt and he was bloody and bruised and a mess, and I was horrified. I began to cry. My other uncle, who was with L.D., said Marla just shut up and go get your dad," she said.
Marla Cooper is now convinced there was not a car accident, but that her uncle was injured crashing to earth in a parachute. She says that she also remembers a discussion about the money that day.
"I heard my uncle say we did it, our money problems are over, we hijacked an airplane," she said.
It later became clear, however, that there was no money. It is believed that the hijacker lost much of the cash as he came crashing down.
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."