18/01/2017 16:12 GMT | Updated 18/01/2017 16:21 GMT

DB Cooper: New Evidence Emerges As Citizen Sleuths Analyse Skyjacker's Tie

Intriguing results from a band of amateur scientists.

Six months after the FBI admitted defeat in the infamous DB Cooper skyjacking case, intriguing new evidence has emerged.

The agency had described its probe into the mystery of Cooper, who vanished out the back of a Boeing 727 into a freezing rain wearing a business suit, a parachute and a pack with $200,000 in cash, as one of the longest and most exhaustive in its history.

The agency said it will preserve evidence from the case at its Washington, DC headquarters, but it won’t act on further tips unless people find parachutes or Cooper’s money.

Handout . / Reuters
Artist sketches released by the FBI of a man calling himself DB Cooper, who vanished in 1971 with $200,000 in stolen cash after hijacking a commercial airliner

Now a group of amateur scientists known as Citizen Sleuths have analysed microscopic particles from Cooper’s clip-on tie, which he left on his seat before exiting the aircraft.

It revealed the presence of elements including cerium, strontium sulphide and pure titanium, which the group describes as “rare and exotic” at the time, thus narrowing the field of possible suspects.

One place they were being used was for Boeing’s high-tech Super Sonic Transport plane, which was developed with government funding in the 1960s and 1970s.

Handout/ Reuters
Some of the stolen $20 bills recovered after DB Cooper leapt from the plane 

Lead researcher Tom Kaye told King5 he wonders if Cooper was a Boeing employee or contractor who wore a tie to work.

He said: “The tie went with him into these manufacturing environments, for sure, so he was not one of the people running these (manufacturing machines). He was either an engineer or a manager in one of the plants.”

Kaye is appealing to the public, particularly those with experience in the aerospace industry in the Northwest around the time, to contact Citizen Sleuths with theories on what those materials could have been used for.

He added: “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.’”

Associated Press
This is the Northwest Airlines jet hijacked by DB Cooper in 1971

On 24 November, 1971, the night before Thanksgiving, a man described as being in his mid-40s with dark sunglasses and an olive complexion boarded a flight from Portland, Oregon, to Seattle-Tacoma International Airport.

He bought his $20 ticket under the name “Dan Cooper,” but an early wire-service report misidentified him as “DB Cooper,” and the name stuck.

Sitting in the rear of the plane, he handed a note to a flight attendant after takeoff. “Miss, I have a bomb and would like you to sit by me,” it said.

The man demanded $200,000 in cash plus four parachutes.

He received them at Sea-Tac, where he released the 36 passengers and two of the flight attendants. The plane took off again at his direction, heading slowly to Reno, Nevada, at the low height of 10,000 feet. Somewhere, apparently over southwestern Washington, Cooper lowered the aircraft’s rear stairs and jumped.

He was never found, but a boy digging on a Columbia River beach in 1980 discovered three bundles of weathered $20 bills - nearly $6,000 in all. It was Cooper’s cash, according to the serial numbers.

Over the years, the FBI and amateur sleuths have examined innumerable theories about Cooper’s identity and fate, from accounts of unexplained wealth to purported discoveries of his parachute to potential matches of the agency’s composite sketch of the suspect.

The 40th anniversary of the case in 2011 brought a wave of attention.

An Oklahoma woman told the FBI about a late uncle who showed up to Thanksgiving dinner in Oregon that year with serious injuries. A team that included a paleontologist from Seattle’s Burke Museum announced that particles of pure titanium found in the hijacker’s clip-on tie suggest he worked in the chemical industry or at a company that manufactured titanium.

A book that year, “Skyjack: The Hunt for DB Cooper,” by Geoffrey Gray, posited several theories, including that Cooper might have been a transgender mechanic from Washington state, the Associated Press writes.

The most promising leads in recent years have come from amateurs, Gray said - including a notion put forth by a scientific illustrator at the University of Chicago that Cooper might have taken his persona from a French comic called “Dan Cooper,” a series about a test pilot in the Royal Canadian Air Force.

In one issue from 1963, the character boards an airliner wearing a dark suit and a mask over his eyes and sits in the back of the plane. He demands to be given a briefcase that’s in the cockpit, and then, wearing a military parachute, he jumps out - over a wooded area, at night, in the rain.

The comic was never translated into English, raising questions about whether the hijacker might have been a francophone.

“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,” Gray said.