In November 1971 a man known as Dan Cooper hijacked a commercial airline and was never seen again.
The mystery has baffled cops in the American states of Oregon and Washington in the north west for more than 50 years.
Little is known about the hijacker or where he ended up but he did escape with a $200,000 (£160,000) ransom which is more than $1million (£802,000) today.
How did he get away with it?
Cooper boarded a Northwest Orient Airlines flight in Portland on November 24, 1971.
The plane was only travelling a short journey to Seattle-Tacoma International Airport which is 160 miles away.
After take off the hijacker ordered a drink and told the flight attendant he had a bomb on board and wanted money and parachutes at the destination airport.
The demands were met and the aircraft’s passengers safely disembarked.
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Cooper then instructed the crew to fly him to Mexico City via Reno, Nevada for a fuel stop.
On the way there he parachuted out of the plane over southwest Washington with the money and disappeared.
Even Cooper’s identity was never confirmed by investigators who searched in vain for years to track him down.
The first big lead in the case only came nine years later in 1980 when a boy found $5,800 (£4,600) of damaged twenties along the Columbia River in Vancouver.
The cash was found by eight-year-old Brian Ingram and confirmed to be that of the ransom given to Dan Cooper.
No more of the cash has ever been discovered.
According to the U.S. Bureau of Labor Statistics, the ransom Cooper received would be worth $1,352,513.45 (£1,085,432) today.
The incident has become folklore in the northwest of America and fascinated residents.
It also led to a boom in visitors to the Clark County Historical Museum.
Katie Bush, the museum’s public historian, told OPB that Saturdays are usually less busy days but with the hijacking anniversary last year, “people are asking about him a lot.”
Speaking about why so many people are intrigued by the case, Bush said with a laugh: “Everyone’s hoping that they can find some of his money out there.
“Maybe someday it will come floating down the Columbia and we’ll find it. We all want to find a bag of money on the side of the road.”
Another lead came in 2000 when a skull which was found near Washington 20 years previously in 1981 came to the spotlight as investigators questioned whether it belonged to Dan Cooper.
However, it was later determined that it belonged to someone else.
The FBI investigation into the hijacking was concluded in 2016 after a 45-year hunt.
A statement on the bureau’s website read: “In order to solve a case, the FBI must prove culpability beyond a reasonable doubt, and, unfortunately, none of the well-meaning tips or applications of new investigative technology have yielded the necessary proof.”
But that hasn’t stopped others continue to tell the story of Dan Cooper and his mysterious hijacking.
A Hollywood film is in the works titled ‘Nod If You Understand’ telling the story of flight attendant Tina Mucklow who was on the plane with Cooper.
The film director Amber Sealey told Deadline : “This hijacking is an important piece of US history, much obsessed over, and yet there are huge parts of the story that have never been shown before.”
The story has also inspired several commercial ventures with an escape room opening themed on the incident and even a brewery in Vancouver.
There is also an annual convention named CooperCon where theorists gather to discuss what Cooper’s eventual fate may have been.
However the story ended, the truth has so far evaded law enforcement and the public’s fascination with the case will continue.
