It is a case too baffling for the most experienced investigators in America.

But even after the FBI shut the book on the mysterious case of DB Cooper, armchair detectives have refused to give up the search for answers.

They've continued to investigate who this well-dressed businessman was, why he hijacked a US domestic flight 45 years ago, and how he then disappeared without a trace.

And now they've stumbled across an exciting new lead.

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Three scientists that are part of a citizen investigation into the case have revealed their analysis on a necktie likely left behind by Cooper when he jumped out of a Boeing 727 jet in 1971, could narrow down the hijacker's elusive identity, and blow the now-closed case - the only unsolved hijacking in America's history - wide open.

THE HIJACKING

The plane was refuelled at Seattle. Photo / AP
The plane was refuelled at Seattle. Photo / AP

The man known as DB Cooper boarded a Seattle-bound Northwest Orient Airlines flight in Portland, Oregon, on November 24, 1971. He had a suitcase with him and sat in seat 18E.

Aged in his 40s and dressed in a smart business suit, Cooper calmly ordered a bourbon, lit a cigarette, and put on dark sunglasses. Shortly after takeoff, he handed a note to the flight attendant nearest to him, 23-year-old Florence Schaffner, which said the aircraft was being hijacked and Cooper had a bomb in his suitcase.

Cooper sat next to a terrified Schaffner and detailed his demands: four parachutes, a fuel truck on standby at Seattle airport - where the plane was headed - and a second flight to Mexico City. He also demanded $200,000 in cash, estimated to be about $A1.6 million today.

Cooper opened his suitcase to reveal what Schaffner would later describe as a battery and eight red cylinders with wires attached.

The attendant relayed Cooper's demands to the flight crew, and the crew informed the police. As the plane approached Seattle, the FBI rushed to meet each of Cooper's demands.

Then things took another unexpected turn. After the Boeing jet touched down at Seattle airport, Cooper allowed the 36 other passengers and some crew members to disembark. He stayed on the plane and told the pilot to fly him to Mexico, slowly, while remaining below 10,000 feet.

As the jet crept its way towards Reno, Nevada, for a planned fuel stop en route to Mexico, Cooper opened a rear door and, with the $200,000 cash and a parachute, jumped out.

THE FUTILE INVESTIGATION

An artist's sketch of D.B. Cooper was based on recollections of passengers and crew. Photo / AP
An artist's sketch of D.B. Cooper was based on recollections of passengers and crew. Photo / AP

In the 45 years since that event, no trace of Cooper's body has been found, fuelling speculation he survived the jump.

Hundreds of theories emerged over the years. Some people said Cooper was a former paratrooper. Various families claimed Cooper was their relative. Someone pointed out the entire hijacking was eerily similar to the events of a 1963 French comic called Dan Cooper.

For a while the primary suspect was John List, who murdered his whole family days before the hijacking. Other former suspects included a Vietnam veteran-turned-Catholic priest, a transgender mechanic, and a leather worker posthumously accused by his niece.

In 1980 a young boy digging in sand north of Portland unearthed bundles of cash that matched the serial numbers of Cooper's ransom money. The cult status surrounding Cooper grew, but the mystery remained.

"The fascination with Cooper has survived not because of the FBI investigation, but because he was able to do something that not only captured the public imagination, but also maintained a sense of mystery in the world," author Geoffrey Gray wrote in his book Skyjack: The Hunt for DB Cooper.

But no theory nor claim could confirm who Cooper was, why he hijacked the flight, and if he survived his daring jump.

In July last year, after "one of the longest and most exhaustive investigations in our history", the FBI admitted defeat.

"During the course of the 45-year [hijacking] investigation, the FBI exhaustively reviewed all credible leads, co-ordinated between multiple field offices to conduct searches, collected all available evidence, and interviewed all identified witnesses," the bureau said.

"Although the FBI appreciated the immense number of tips provided by members of the public, none to date have resulted in a definitive identification of the hijacker."

America's most experienced investigators had abandoned the case, but its armchair detectives refused to give up.

THE NEW DEVELOPMENT

A tie left behind in Cooper's aeroplane seat may have revealed his employer. Photo / Supplied
A tie left behind in Cooper's aeroplane seat may have revealed his employer. Photo / Supplied

This week, scientists working for Citizen Sleuths - a group that took up its own investigation into the Cooper case in 2007 - claimed they had a breakthrough.

For the past decade, Citizen Sleuths have been working away on the case, publishing their exhaustive research online.

Key members of the Citizen Sleuths team are scientists, including principal investigator Tom Kaye, a palaeontologist and spectroscopy specialist, as well as experts in optical miscrosopy, metallurgical engineering and biological illustration.

Their work has attracted the attention of an impressed FBI, which some years ago let them examine clues, including a tie.

The black, JC Penney clip-on necktie had been left behind on Cooper's seat - 18E.

And that tie, the citizen sleuths said, has given them a solid clue about the man's identity.

"A tie is one of the only articles of clothing that isn't washed on a regular basis," the group explained on their website.

"It picks up dirt and grime just like any other piece of clothing, but that accumulation never truly gets 'reset' in the washing machine. Each of those particles comes from something and somewhere and can tell a story if the proper instruments like electron microscopes are used."

The scientists used a powerful electron microscope to find more than 100,000 particles of "rare earth elements" on the tie, including pure titanium, which most caught their eye.

They said titanium was a rare metal in 1971, and linked Cooper to a "limited number of managers or engineers in the titanium field that would wear ties to work".

Based on this finding, the scientists said they believed Cooper worked for Boeing - the maker of the very plane he hijacked.

At the time, Boeing happened to be working on a Super Sonic Transport plane that used those elements.

"The tie went with him into these manufacturing environments, for sure, so he was not one of the people running these [manufacturing machines]," the group's lead researcher, Tom Kaye, told Seattle's King 5 News.

"He was either an engineer or a manager in one of the plants."

The group is now asking for public help to develop their theory Cooper was a Boeing employee.

They've detailed the list of found particles on their website and hope other citizen investigators detectives can help piece together the mystery of DB Cooper.

"Someone may be able to look at those particles and say 'Oh my gosh. I know what that means having those particles on the tie'," Kaye said.

The FBI has said it would preserve evidence from the case at its Washington, DC headquarters but won't act on further tips unless Cooper's money or parachutes were found.

In the meantime, another amateur sleuth is working away on the case - Geoffrey Gray, who wrote the book about the Cooper hijacking mystery in 2011.

He has made published hundreds of FBI files related to the case his online magazine True Ink in the hope opening the case to the public would help finally solve the baffling case.

"We're trying to solve one of the greatest unsolved mysteries of our time," he said. "And we need your help."