A letter newly released from the FBI's archives may prove that DB Cooper - the 1971 hijacker last seen leaping out of a plane with a fortune in cash - survived his apparent death, the Daily Mail reports.
The letter, which was sent 17 days after the hijacking appears to contain information that was not released into the public domain until 13 years later.
If that's the case it might reveal not only that Cooper lived to tell the tale of his extraordinary heist, but that the FBI covered it up to hide their embarrassment at his escape, the leader of a team of private investigators told the Daily Mail.
The hijacking occurred on November 24, 1971, when a man calling himself Dan Cooper boarded a Boeing 727 at Portland Airport and announced shortly after takeoff that he was holding a bomb.
The plane was grounded at Seattle-Tacoma airport until authorities produced $200,000 in cash - the equivalent of $1,213,226 today - and Cooper released the passengers.
But he kept some of the crew on board and forced them to take off before opening up the back of the plane and parachuting out over the forests of Washington, clutching the money and clad only in a suit.
The FBI ultimately concluded that Cooper - whose real name is unknown - died of exposure, and last year they closed the case.
But a team of around 40 private investigators led by TV and film executive Thomas Colbert and his wife Dawna believe that Cooper is alive - and told the Daily Mail they have the evidence to prove it.
They successfully sued the FBI and have obtained reams of previously undisclosed information about the case, including a series of five letters purportedly written by Cooper to the authorities.
Four of those letters were already known to the public, but now a fifth letter has been produced. And Colbert says it proves the existence of an FBI cover-up.
Copies of the typewritten letter were sent to The Seattle Times, The New York Times, The LA Times and The Washington Post.
It begins "I knew from the start that I wouldn't be caught," and apparently sees Cooper explaining exactly why the police will never catch him.
Among those reasons is that "I left no fingerprints".
For Colbert, who obtained the letter in his most recent Freedom of Information Act (FOIA) package, that admission was an eye-opener.
"This was the biggest secret of the whole investigation, not revealed until two FBI agents wrote books in 1984 and 1991 and discreetly mentioned it,' Colbert explains.
"No fingerprints of value" were recovered anywhere Cooper was in the aircraft, including on his drinking cup. The only man outside the Bureau to know that would be Cooper.
"And, I might add, none were found on this fifth DBC letter or the earlier four. Highly unlikely we have two separate Coopers with the same meticulous habit."
The letter also ends "Thank you for your attention" - a phrase similar to the "Thank you" message that conclude the first two letters.
That constituted a "thought pattern" indicating that the letters were likely all written by the same man, according to retired FBI Special Agent Dorwin 'Dory' Schreuder, who worked the Cooper case in the 1980s and is now on Colbert's team.
Were these clues? The FBI seemed to think so, Colbert said, as it went to all four of the papers the letter was sent to and retrieved their copies before burying them in their files.
The scan of the letter provided to Colbert by the FBI also has an 'EVIDENCE' stamp - unlike its predecessors, which suggests that it was being taken more seriously by the authorities.
The FBI also began to look for a bald man for the first time after the letter was sent, matching its claim that Cooper "wore a toupee".
Descriptions of Cooper by those who saw him in the plane also suggest that he may have indeed been wearing a wig and putty - or at least makeup - as a disguise.
And yet, said Colbert, the letter was never made public knowledge beyond a couple of brief reports in the newspapers that played it off as a prank.
In fact, he said the FBI continued to try to hide the letter to the extent that an FBI memo about the letter given to the team in last month's FOIA dump had the only reference to Cooper redacted from it.
Colbert added that the other four letters, though officially 'discounted' by the FBI, were taken seriously at the time.
So much so that J Edgar Hoover himself was keeping tabs on the search for the letters' author. He even had the printing on their envelopes, along with the only letter that was handwritten compared with those of the Zodiac Killer (who, thankfully, was not a match).
Colbert added that he and his wife hired two forensics experts - one a past president of the American Society of Questioned Document Examiners, the other an ex-FBI agent - to examine the one handwritten note.
That handwriting is so similar to the writing on Cooper's boarding card, the two men said, that it was "likely" that they "were written by one person".
Colbert also feels that the FBI has been too hasty to wrap up the investigation, and has been unduly reluctant to reopen it.
In August, his team found what they believe to be a parachute strap and foam padding from skydiver's backpack in the forests near Cooper's alleged jump location. They turned over the two items, along with the dig site itself, to the FBI.
In addition, Colbert said two potential escape partners - both still alive - were identified during the research. The contacts for those men were likewise delivered to the feds.
But after three months of silence, the team's attorney, Mark Zaid, believes the Bureau hasn't taken action on any of the team's efforts.
Colbert's team is now resuming digging in the area without federal help.
When contacted, a spokesperson for the FBI's Seattle Office, Ayn S Dietrich-Williams, said: "In July 2016, the FBI redirected resources allocated to the DB Cooper (NORJAK) case in order to focus on other investigative priorities.
"However, we asked that individuals contact the FBI if physical evidence emerges related specifically to the parachutes or the money taken by the hijacker.
"It would be inappropriate to comment on any specific tips provided to us in this case, however our continued posture is to review any physical items provided and pursue follow-up actions, as appropriate."
Dietrich-Williams declined to answer any further questions about the suspected strap or dig site.
But if Cooper is alive, how did he escape exposure in the wild - and where is he now?
Colbert has dug up contemporary newspaper reports, eyewitness accounts and court-ordered FBI agent field notes that suggest other men were seen in a small plane in another airfield around the time of the hijacking.
He believes they picked up Cooper after he landed and flew under the radar to drop him off safely so that he could make his getaway.
That story, he says, matches a claim made by a man at a pilot's club in at the now-closed Evergreen Field in Vancouver, who said in 1997 he heard the secret details of the getaway flight from an older flyer.
That flyer is now believed by Colbert to have been one of Cooper's escape partners.
And what of Cooper himself? Colbert believes he was a Vietnam veteran called Robert W Rackstraw Sr, now 74.
Rackstraw had an illustrious military career, being a pilot in the 1st Cavalry Division, one of the first major American air assault divisions.
It was there that Rackstraw learned to parachute, and was given two Distinguished Flying Crosses for his performance while in the air - but he was kicked out of the army after they discovered he'd lied about dropping out of high school and attending two colleges.
Colbert believes the military gave him all the skills he needed to pull off the extraordinary heist.
He also notes that the first and fourth letters sent by 'Cooper' were sent from Sacramento and Oakdale, California, both close to Rackstraw's family home.
In 1977, six years after the hijacking, Rackstraw was suspected of kiting checks for $75,000, but fled before arrest and went to Iran to teach the Shah's men how to fly helicopters.
Meanwhile, back at home, police raided his storage units and found 14 rifles and 150lbs of dynamite.
He lost his his Iran chopper job and he was brought back the USA, where he was arrested for fraud and the murder of his stepfather. Philip Rackstraw was found in the grounds of his parents' home with two bullets in his head.
Rackstraw was acquitted of murder and made bail on the fraud charge. Then he faked his death, pretending to crash his plane in the ocean at Monterey Bay, California.
He was found and taken in by the FBI, who believed he might be Cooper, but a lack of evidence and the sudden discovery of some of the hijack money in Washington - planted, Colbert says, by an accomplice - led to his release.
Rackstraw made a plea deal and after serving a year in prison for his Stockton convictions, he moved to Riverside, California.
There he taught a law course in mediation before retiring to his yacht, 'Poverty sucks'.
Is Rackstraw the greatest air criminal in American history? His lawyer has previously called the claims "the stupidest thing I ever heard".
Attempts to reach out to the lawyer again for this story were unsuccessful.