A woman claiming to be the niece of legendary skyjacker D.B. Cooper says she is the source of the FBI's new interest in the 40-year-old mystery.

In an interview that aired Wednesday on ABC's "Good Morning America," Marla Cooper said she had provided the FBI with evidence -- including a photo and a guitar strap -- that could help prove her uncle, Lynn Doyle Cooper, was the man who parachuted out of a Boeing 727 airliner with $200,000 and vanished. She said Cooper survived the plunge but died in 1999.

"I'm certain he was my uncle, Lynn Doyle Cooper. Who we called L.D. Cooper," she told ABC News.

The incident unfolded on Nov. 24, 1971, when a man calling himself Dan Cooper hijacked a Northwest Orient Airlines jetliner flying from Seattle to Portland, Ore. The hijacker threatened to blow up the plane, forced it to land and demanded $200,000 ransom and a parachute. After the plane took off, he lowered the rear steps of the jet and leaped out over the Pacific Northwest, and a legend was born.

The FBI mounted a massive manhunt at the time and has been pursuing the case ever since, but the trail had grown cold until this week, when the FBI said it had been given "credible evidence" in the case.

That evidence, according to ABC, included a man's name and guitar strap, provided by Marla Cooper. No fingerprints have been found on the guitar strap, ABC said, citing an anonymous source.

Cooper told ABC that as an 8-year-old girl she remembers her two uncles plotting something suspicious on Thanksgiving Day 1971 at her grandmother's house in Sisters, Ore., near the area where D.B. Cooper jumped out of the plane.

"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."

The men did not return, she said, and the hijacking took place the next day.

She said her uncle came home that day wearing a bloody T-shirt.

"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 in the ABC interview.

She says that she also remembers her uncles talking about the money that day.

"I heard my uncle say ‘we did it, our money problems are over, we hijacked an airplane,'" she told ABC.

ABC's full story: Woman claims D.B. Cooper was her uncle.