It is one of the biggest mysteries in US criminal history: just what happened to DB Cooper, the man who hijacked an airplane before leaping out in mid-air with $200,000 in cash?

Now, more than 50 years later, the infamous crime may have been solved, after a pair of siblings came forward to claim they had found the parachute used in the hijacking, in their mother’s shed, and that Cooper was their father.

Chanté and Rick McCoy III say their father, Richard McCoy Jr, was the man who identified himself as Dan Cooper when he boarded a Northwest Orient Airlines jetliner from Portland to Seattle in November 1971.

Cooper, or perhaps McCoy, proceeded to order a bourbon and soda before handing a note to a flight attendant that said he had a bomb in his briefcase.

“Miss, I have a bomb and would like you to sit by me,” the note said.

When the plane arrived in Seattle, Cooper collected $200,000 in ransom money, along with four parachutes, and released the passengers. He then ordered the flight crew to head for Mexico City, via Reno, Nevada, but 30 minutes after takeoff, Cooper jumped out of the airplane somewhere over south-west Washington.

The hijacking baffled the FBI, who spent 45 years investigating before officially closing the case in 2016. It also caught the attention of amateur sleuths, particularly after about $5,800 of the ransom money was found near Vancouver, Washington, in 1980.

In November, Dan Gryder, a retired pilot who has spent 20 years investigating the case, told the Cowboy State Daily that the FBI was re-investigating the Cooper case, after the discovery of the parachute in the McCoys’ mother’s shed.

“That rig is literally one in a billion,” Gryder said of the parachute, according to the Cowboy State Daily. He said FBI agents had visited the property of the McCoys’ mother, Karen, who died in 2020, last year. Agents searched “every nook and cranny”, according to Gryder, and the McCoys handed over the parachute.

Gryder released two videos in 2021 and 2022 on his YouTube channel Probable Cause, documenting the McCoys’ claims. In Gryder’s latest video, published on 18 November, he claims the FBI effectively re-opened its investigation after contacting him in late 2023. Gryder claims that the FBI, which is in possession of the McCoy parachute, is now searching for “a positive DNA connection between McCoy’s DNA, and the Cooper DNA left on board the aircraft”.

The FBI did not immediately respond to a request for comment.

The suggestion that Richard McCoy may have hijacked the Northwest Orient Airlines is not as outrageous as it may seem.

McCoy, a former military helicopter pilot who served in the Vietnam war, was among a number of suspects investigated by the FBI after he hijacked a plane on 7 April 1972, leaping out of the aircraft with $500,000 cash, over Provo, Utah. McCoy was arrested two days later and sentenced to 45 years in prison, but he escaped in 1974 – after three months on the run he was killed by an FBI agent.

Perhaps McCoy died hiding the secret of the DB Cooper hijacking with him – and 50 years on, the truth may finally have come to light.