A woman has claimed her uncle was the legendary "D.B. Cooper", the man behind an unsolved 1970s skyjacking, and she is cooperating with investigators to prove it, ABC News reported Wednesday.

Marla Cooper has given the FBI a guitar strap and a photo of a man whom she calls Lynn Doyle Cooper, and who she believes hijacked a Northwest Orient Airlines jet in 1971 and then parachuted to the ground with $200,000 in cash.

"I'm certain he was my uncle, Lynn Doyle Cooper," she told ABC News.

The woman said she recalled her two uncles hatching a plot at her grandmother's house in the western US state of Oregon, not far from the spot where the hijacker jumped out of the Boeing 727 at night.

"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. The next day, the plane was hijacked.

Her uncle came home "bloody and bruised and a mess," claiming to have been in a car accident, but she now believes he was injured in the parachute jump. She also told ABC News she heard him speak about money.

"I heard my uncle say, 'We did it, our money problems are over, we hijacked an airplane,'" she said, adding she never saw her uncle again, and was told he had died in 1999.

The man who hijacked a Northwest Orient Airlines jet on November 24, 1971, called himself "Dan Cooper," but even after FBI agents investigated -- and cleared -- a man named D.B. Cooper, the latter name stuck in media accounts.

The hijacker ordered the jet -- which was on a short flight from Portland, Oregon to Seattle, Washington -- to land after claiming to have a bomb and showing a flight attendant a briefcase filled with red cylinders and wires.

When the plane landed in Seattle, the man traded the passengers for $200,000 and a parachute before ordering the crew to fly him south to Mexico.

About 30 minutes later, the hijacker jumped at an altitude of 10,000 feet. He was never found.

Marla Cooper said she recalled conversations with both of her parents which suggested her uncle was the culprit.

"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.

The FBI said Monday they were looking into a promising lead in the case, and were working with a suspect's family to match his fingerprints to those found in the plane.

Sources told ABC News that Marla Cooper had offered evidence.