A woman who claims to be the niece of airplane hijacker D.B. Cooper said she had a role in renewing the FBI's probe into the 40-year-old case.

Marla Cooper told ABC News she is sure that her uncle Lynn Doyle Cooper is the one who pulled off the notorious 1971 hijacking by threatening to blow up a commercial jetliner before leaping from the airplane with a parachute and $200,000 in ransom.

Marla Cooper said she is certain after clarifying childhood memories about the incident and through recent conversations with her parents.

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

Marla recalls being 8 years old and hearing her two uncles making suspicious plans at her grandmother's Oregon home near where D.B. Cooper jumped.

"My two uncles, who I only saw at holiday time, were planning something very mischievous," she told ABC News. "I was watching them using some very expensive walkie-talkies that they had purchased. They left to supposedly go turkey hunting, and Thanksgiving morning I was waiting for them to return."

One day later, Northwest Orient flight 305 was hijacked, and her uncle L.D. Cooper came home and said he was 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," Marla said, doubting there was ever a car accident, but that her uncle was the parachuting criminal.

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

ABC News said it is believed that the D.B. Cooper lost much of the money as he crashed down from the jet.

Marla has provided FBI agents with a guitar strap and a Christmas photograph of her uncle picture with the same strap.

Federal agents are now searching an item they got for fingerprints at their forensic laboratory in Quantico, Va. This will be checked against partial prints found from the hijacking scene. Meanwhile, the FBI is hunting for proof that L.D. Cooper was on the jet in 1971.

D.B. Cooper's identity has been a mystery since 1971, after the man who called himself Dan Cooper hijacked a Northwest Orient Airlines bound for Seattle from Portland, and jumped from the back of the jet thousands of feet over the Pacific Northwest.

The FBI recently announced that it was following a new lead in the case.

FBI agent Fred Gutt told the Seattle Times that the agency was tipped by a retired law enforcement official that a man who died 10 years ago in the Pacific Northwest was Cooper. Agents have requested the man's personal effects and are trying to find fingerprints or DNA to compare with items the hijacker left behind.

A new book called "Skyjack," which is based on the unsolved hijacking, is set to come out on Aug. 9 -- about a week after the FBI announced the new lead.

Its author Geoffrey Gray told NBC News that he doesn't think the prints found on the airplane can provide a good match.

"The problem with the print is just that there (are) just too many prints. And the prints that the FBI has been able to find aren't necessarily good ones," Gray said.

More than 1,000 leads have been checked since Cooper leaped out of the jetliner on Nov. 24, 1971, over the Pacific Northwest.
Watch Marla Cooper's interview here.