Aug. 9, 2011 -- Perhaps the last, best forensic evidence in the case of infamous hijacker D.B. Cooper has proved to be just another inconclusive, frustrating lead in the decades-long investigation.

The FBI said the DNA found on the hijacker's clip-on tie and DNA taken from the daughter of potential suspect Lynn D. Cooper is "not a match." However, FBI Special Agent Fred Gutt said, "That doesn't suggest that the current lead is a dead end."

The late Lynn D. Cooper surfaced publicly as a possible suspect in the 1971 hijacking last week. His niece, Marla Cooper, told ABC News that she believed her uncle was the man who had hijacked a Northwest Orient jet flying to Seattle. The man had threatened to blow up the plane and its passengers unless he received a ransom payment and a parachute. He released the passengers, told the pilot to fly him to Mexico, and jumped from the back stairs of the plane somewhere over Washington state. He parachuted to the ground with $200,000, and has never been identified by authorities.

Over the years, the story of D.B. Cooper became the stuff of folk songs, books and movies. But the cold case got warm again earlier this year when it was reignited by Marla Cooper. She provided the FBI with information about her uncle, whom the family called "L.D.," and a guitar strap that may have carried his DNA. Tests later found that the guitar strap did not have any DNA remnants. Cooper also put the bureau in touch with L.D.'s widow and her daughter. The FBI took a DNA sample from the daughter to compare to the DNA on the clip-on tie, but the DNA did not match up.

FBI Special Agent Fred Gutt told ABC News, "It's possible that the DNA sample taken off the tie was not from the hijacker. There are questions about the tie.It may have been borrowed, or purchased used. The DNA may be from someone else," Gutt said. Gutt also said, "The tie had two small DNA samples, and one large sample lifted off in 2000-2001. It's difficult to draw firm conclusions from these samples."

When asked if this "no match" finding damaged Marla Cooper's credibility, Gutt replied it did not. "We haven't come up with anything that is inconsistent with her story," he said. Marla Cooper also passed an FBI polygraph test about the case.

What the FBI really needs to make the case, Gutt said, is a fingerprint from L.D. Cooper to match against fingerprints found on the hijacked plane. "We are working with surviving family members to try to identify objects he may have handled," Gutt said.

Since L.D. Cooper died in 1999, that may prove to be an impossible task, but the FBI is planning to revisit Marcia Cooper, L.D.'s widow, and other relatives, to see if they can find anything that may still carry L.D.'s prints. "Without fingerprints," Gutt said, "we just have circumstantial evidence."

Part of that circumstantial evidence is the fact that L.D. Cooper is a perfect fit for the profile of the hijacker developed by the FBI years ago. The FBI profile saw the hijacker as a former military man who lived in the Northwest, and was a capable outdoorsman. In addition, they believe he had to be a fan of the 1960s French comic book character Dan Cooper -- the character used to create the famous hijacking alias D.B. Cooper.

L.D. Cooper was a timber faller, or lumberjack for part of his young life, and lived in Sisters, Ore. He served in the Korean War, and was obsessed with the Dan Cooper comics, according to his niece Marla Cooper. The possibility of L.D. being the real hijacker is buttressed by the recollections of Marla and her mother, Grace Hailey.

"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 immediate got cut off again."

Marla Cooper said that as an 8-year-old she recalled her two uncles planning something suspicious at her grandmother's house in Sisters -- not far from where D.B. Cooper jumped from a plane with $200,000 in cash one day later.

"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. She said she now believes the men were practicing for the post-hijacking recovery operation. "They left to supposedly go turkey hunting, and Thanksgiving morning I was waiting for them to return."

A day later, Northwest Orient flight 305 was hijacked, and her uncle L.D. Cooper came home claiming to have been 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," she said.

Marla Cooper is now convinced there was not a car accident, but that her uncle was injured crashing to earth in a parachute. She says that she also remembers a discussion about the money that day.

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

It later became clear, however, that there was no money. It is believed that the hijacker lost much of the cash as he came crashing down, and some of the hijacking money was recovered in 1980 when it washed ashore on the banks of the Columbia River.

Marla Cooper said that her two uncles wanted to return to search for the cash, but her father refused. She believes this was because the FBI search was just beginning to take shape.

After that Thanksgiving Day, she only recalls seeing her uncle once more, at Christmas, in 1972, when the picture of him with the guitar strap was taken.

FBI Agent Gutt said the investigation would continue, and would focus on the attempt to find a fingerprint of the late L.D. Cooper to match to the fingerprints the hijacker left on the plane. Without that, the legend of D.B.Cooper may remain a unsolved mystery.