D.B. Cooper Mystery: Marla Cooper's Mom Comes Forward About Brother-in-law

Aug. 4, 2011— -- The mother of Marla Cooper, the woman claiming to be the niece of D.B. Cooper, also believes that her brother-in-law is the infamous skyjacker, and has provided further details about the man who could be the daring culprit in the decades-old case.

Marla Cooper recently came forward to the FBI with evidence that she believes proves that her uncle Lynn Doyle Cooper is the famed D.B. Cooper, the man hijacked and threatened to blow up a commercial plane flying to Seattle in 1971, then parachuted to the ground with $200,000 in hand.

Her mother, Grace Hailey, told ABC News that she doesn't remember much about that Thanksgiving in 1971 where her brother-in-law returned to the house in Sisters, Oregon, but she believes he could be the hijacker. Hailey's statements are one reason why the FBI thinks the tip from Marla Cooper is credible.

"I've always had a gut feeling it was L.D.," Hailey told ABC News. "I think it was more what I didn't know is what made me suspicious than what I did know, because whenever the topic came up it immediately got cut off again."

Hailey says that L.D. grew up in Sisters and was familiar with the area where the hijacker jumped -- a fact that is consistent with the FBI's theory that D.B. Cooper knew the Pacific Northwest. He was also a war veteran, which matches the theory that the hijacker had a military background.

L.D. Cooper was a logger and an outdoorsman -- tough enough, Hailey believes, to leap out of plane into the wilderness. His soundness of mind, her daughter feels, is a different story.

"I think a man who was willing to jump out of airplane, in the middle of an ice storm over a forest, doesn't have his wits about him," Cooper told ABC News.

Marla Cooper has 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.

She is working on a book about her belief that her uncle is the hijacker, but that is not her main motivation for coming forward.

"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."

Marla Cooper told ABC News 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.

Steve Dean, the assistant special agent in charge of the criminal division of the Seattle FBI office, confirmed Wednesday to the Seattle Times that Marla Cooper had contacted the bureau and turned over items to assist in the investigation.

She says that the family saw L.D. the following Christmas in 1972, after his hospital stay -- which was when the picture she provided to the FBI and to ABC News was taken. After this her uncle quickly faded from their lives.

One key reason she is convinced that her uncle was D.B. Cooper is his sudden disappearance from the family after that Thanksgiving.

"I definitely think it was strange that he kind of disappeared like that," she said, adding that she does not think he was a bad man. "I don't think he was evil, and I think he regretted it."

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.