Squeezed into the back row of the cabin on Flight 305, a man sparked a cheap cigarette and ordered a drink – bourbon and coke.

He wore a dark suit, a narrow tie (which would have been stylish if it was a few years earlier), and a pair of aviator sunglasses.

His luggage was a dark attaché case. Inside of it, he told flight attendant Florence Schaffner there was a bomb.

Calling himself Dan Cooper, the hijacker handed Schaffner a note with his demands.

But the flight attendant thought it was a cheesy pick-up attempt and merely pocketed the paper, assuming it was his phone number.

The numbers Cooper had in mind were not for a landline, they were for $200,000 (About AUD$1.7m with inflation).

When the 23-year-old stewardess ignored the note, Cooper insisted: “You’d better look at that note. I have a bomb”.

The sketch of DB Cooper shows a man in a suit who wore a thin tie and dark sunglasses. (FBI)

Inside the paper was a message written in felt pen: “I have a bomb here. I want you to sit beside me”.

Cooper, standing about 180cm tall and aged in his mid-40s, flipped open the latches on his briefcase to show Shaffner he meant business. She saw red cylinders attached to wires and a battery.

And the hijacker had more than $200,000 on his wish-list. He demanded four parachutes and a fuel truck to wait for the plane in Seattle.

“No funny stuff or I’ll do the job,” he told her.

What happened next has baffled the FBI for nearly 50 years.

The hijacking of Northwest Orient Airlines Flight 305 on November 24, 1971 is the only case of its kind never yo be solved in the US.

The hijacking

The well-spoken hijacker was by all accounts a gentleman. He casually smoked Raleigh cigarettes and ordered bourbons, even offering a hefty tip to the flight attendant for his drinks.

His demands were relayed by her to the pilot, William Scott.

The FBI needed time. $200,000 was a lot of cash. And the parachutes were not easy to source. As the plane circled above Seattle, Cooper glanced out the window and said, “Looks like Tacoma down there”.

DB Cooper's tie with a unique tie clip was left behind. And some of the money was found many years later. (FBI)

The plane landed on the Sea-Tac runway two hours late.

Cooper wanted the plane refuelled immediately. He ordered the passengers to disembark. Only the flight crew and Cooper remained on board.

“It shouldn’t take this long,” Cooper said, as he waited to get back in the air.

Their next destination was known only to Cooper, who issued strict instructions. They would fly under 10,000 feet, with wing flaps at fifteen degrees.

He appeared to have extensive aviation knowledge.

At some stage during the flight Cooper strapped his bundles of cash to a parachute.

He put on his own chute and made his way to the rear of the plane, where stairs could be lowered to let passengers off.

Cooper edged down the stairs into the cool air and clouds. It was about -7C at that altitude.

Below him was a dense forest carved open by the powerful Columbia River.

Suddenly the pilot felt a pressure change. It was Cooper leaping into the abyss with his bounty.

Hundreds of law enforcement agents scoured the area they believed Cooper landed in, but to no avail.

At some stage a journalist called Dan Cooper “DB Cooper” and the moniker stuck. But the feds soon realised the name used to book the ticket was a fake. It was all part of the mystery.

(Nine)

The Cooper Hunters

The intrigue of DB Cooper has grown with every year the mastermind has evaded capture.

In November of this year, the first DB Cooper Con brought together amateur sleuths, aviation experts, and a loyal group of DB Cooper hunters known as “Cooperites”.

Eric Ulis told nine.com.au he created DB Cooper Con in preparation for the 50th anniversary of the crime, with this year the 47th.

Ulis has been scouring thousands of pages of FBI documents for a decade, one of many bonified Cooper hunters across the US. Ulis has recently published his own research – DB Cooper: The definitive investigation of Sheridan Peterson .

Ulis admits his focus on the mystery has ended with some “good humoured ribbing” from his girlfriend. But his dedication to the case is a part of his life now.

“I’ve been asked many times, why? And it was such an audacious and such an innovative crime. I think it’s just human nature. You want to see the man you did it.”

“The thing is he got away, that keeps us at arm’s length.”

The suspects

The FBI never got their man, but Ulis is certain he knows who DB Cooper is. And he’s spoken with him many times.

That man is Sheridan Peterson

“Cantankerous is the word I would use to describe him… my relationship with him has been on again off again,” Ulis told nine.com.au.

The parachute and parachute bag used by the hijacker DB Cooper to escape. (FBI)

“He’s obviously aware that I’m looking into him as suspect. About three or four months ago he cut me off completely.”

Peterson, now in his nineties and living in California, was also a key lead for the FBI.

“He may well have been the very first suspect and he still is a suspect 47 years later. They have never been able to exonerate this guy,” Ulis said.

Peterson is a compelling man. He worked for Boeing and had intimate knowledge of the 727 hijacked by Cooper. He was also an experienced skydiver from his time serving in the military.

But he has an alibi – he told the FBI he was living in Nepal in 1971.

He had also lived in Russia, Vietnam, Japan, Hong Kong, the Middle East and China before returning to the US in 1999.

A comparison of the DB Cooper sketch and Sheridan Peterson. (Eric Ulis)

“They were looking for me for 30 years, it was 2001 that they found me,” Peterson told the History Channel in a previous interview.

The FBI came to his apartment in Santa Rosa in California where Special Agent Mary Jean Fryar asked for a DNA swab, and quizzed him if he was the real DB Cooper

“I thought that was the funniest thing I ever heard in my life” Peterson said of the notion he was Cooper.

“I can’t say that he wasn’t (DB Cooper)… I think he was a damn good suspect,” special agent Fryar said.

Nine.com.au has contacted Peterson for comment.

The other guys

In 2016 the FBI announced it was no longer actively investigating the notorious case. But the legion of DB Coopers continues the hunt for the folk legend. These are the main suspects the Cooperites have in their crosshairs.

- Kenneth Christiansen

The former mechanic for Northwest Orient Airlines was also a paratrooper in the military. His own brother thought Kenneth was DB Cooper.

- Lynn Doyle Cooper

Cooper came into the picture in 2011 when Marla Cooper suggested her uncle was the culprit. The FBI considered the tip against the outdoorsman from Oregon credible.

- Richard McCoy