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Seattle • The "credible suspect" the FBI is investigating in the D.B. Cooper skyjacking case is a man named Lynn Doyle Cooper, who reportedly died in 1999.

ABC News first revealed the name Wednesday in an interview with Cooper's niece, Marla Cooper, who said she is cooperating with the FBI.

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

Cooper, citing childhood memories, told ABC News she is convinced her uncle was the man who hijacked a Seattle-bound jet on Thanksgiving Eve 1971 and parachuted over Southwest Washington with $200,000 in cash.

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

Although some investigators concluded the skyjacker died in the jump, a body was never was found in what remains America's only unsolved hijacking.

The FBI said earlier this week that it was investigating "a promising lead" in the case, but would only identify a possible suspect as a man who died more than 10 years ago.

Cooper, whose hometown wasn't revealed, told ABC News that she is working on a book about her uncle, but said that wasn't her primary motivation for coming forward.

She said that she was 8 years old at the time of the fabled skyjacking.

Cooper said she recalled her uncle and a second uncle planning something suspicious at her grandmother's house in Sisters, Ore.

"My two uncles, who I only saw at holiday time, were planning something very mischievous," Cooper 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."

After Northwest Orient Flight 305 was hijacked, L.D. Cooper came home claiming to have been in a car accident, Cooper told ABC News.

"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.

She said she is now convinced the car accident was a ruse and that her uncle was injured in a parachute jump.

Cooper also told ABC News she remembers a discussion about the money.

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

Cooper says that her two uncles wanted to return to search for the cash, apparently because it was lost in the jump. But her father refused, she told ABC News.

She said she believes her father declined because the FBI was beginning to search the area where Cooper was believed to have landed.

Cooper said she never saw her uncle again after that Thanksgiving and was told he died in 1999. She said she believes he lived in the Northwest and had children.

In the ABC News interview, Marla Cooper displayed a 1972 Polaroid picture of her uncle, whom she identified as a Korean War veteran. She said the picture is similar to the composite sketch of the hijacker.

FBI spokesman Fred Gutt said Monday the bureau's Seattle office has been investigating for more than a year a lead that has "more credibility and detail" than other tips.

Gutt said the FBI's vetting of the case warrants further investigation, noting little contradictory information has emerged that would rule out the possible suspect. But he said that doesn't mean the case is about to be solved.

The FBI laboratory has determined that a guitar strap that belonged to the man is not conducive to lifting fingerprints to compare to partial prints found in the plane, Gutt said.

"It doesn't mean it's a dead end," Gutt said, adding that the case agent is working with the man's family to obtain other items with better surfaces to lift fingerprints.

Gutt said Wednesday that the case is not a high priority, but the new information can't be ignored.

Marla Cooper told ABC News that she provided the FBI with the guitar strap and a Christmas photo of a man pictured with the same strap.

She said that two conversations with her parents initially made her suspicious.

Cooper said the first occurred in 1995, just before her father 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.

She said she had difficulty believing her father, but in 2009 the subject 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.

Cooper told ABC News he contacted the FBI "as soon I was sure that what I was remembering were real memories."

"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 had one of the comic books thumbtacked to the wall.

FBI spokeswoman Ayn Sandalo Dietrich said on Sunday the FBI initially received a tip from a member of law enforcement about a credible person with information on the case.

The passenger who jumped from the Northwest Orient Airlines Boeing 727 on Nov. 24, 1971, identified himself as "Dan Cooper." A day after the skyjacking, FBI agents checked out a Portland man with the name "D.B. Cooper." Although that man was quickly cleared, the name stuck in news-media accounts.

The tall, dark-complexioned man paid $20 cash for a one-way flight from Portland to Seattle. The jet was barely in the air before he told a flight attendant he had a bomb and showed her a briefcase holding several red cylinders and a nest of wires.

When the plane landed in Seattle, passengers were exchanged for parachutes and ransom money paid by the airline. The plane took off and headed south toward Mexico, with the hijacker and the flight crew.

About 30 minutes later, a cockpit warning light showed the rear stairway was fully extended. The pilot asked over the intercom, "Is everything OK back there?"

The hijacker yelled back, "No," and bailed out the back from 10,000 feet into freezing darkness.

Authorities estimated he landed near the small community of Ariel, Cowlitz County, east of Woodland, in a rugged, wooded region.

The man was never found, and only $5,800 of the ransom money—whose serial numbers the FBI had recorded — turned up when a child digging in a sandbar on the north bank of the Columbia River west of Vancouver in 1980 unearthed a bundle of $20 bills.