For decades, both the FBI and amateur investigators have been preoccupied with the identity of “Dan Cooper,” a mysterious passenger mistakenly reported by journalists as "D.B. Cooper" who boarded a flight from Portland to Seattle on November 24, 1971. Without appearing frantic or violent, Cooper informed the crew he had a bomb and demanded $200,000 in ransom. After making the pilots stop for fuel and then lift off again, the skyjacker collected his money and parachuted out of the plane, never to be seen or heard from again.

According to one Cooper devotee, that might not be exactly true. Tom Colbert has led a team of amateur investigators looking into the case and made headlines last year after acquiring some of the closed portions of the FBI’s file via a freedom of information lawsuit. According to Colbert, a letter purportedly written by Cooper and sent to the Oregonian shortly after the crime reveals a “confession” hidden in code. The man’s identity, Colbert claims, is that of Robert Rackstraw, a Vietnam veteran who is now 74 years old and living in San Diego.

“I want out of the system and saw a way through good ole Unk,” the letter read. “Now it is Uncle’s turn to weep and pay one of it’s [sic] own some cash for a change. (And please tell the lackey cops D.B. Cooper is not my real name).”

Colbert showed the letter to Rick Sherwood, a former codebreaker for the now-defunct Army Security Agency. Sherwood maintains the repetitive phrasing of Unk and other words corresponds with a simple letter-to-number code that, when broken, reveals the sentence “I am 1st LT Robert Rackstraw.”

Another letter uncovered in the FBI’s files earlier this year contains a numerical sequence that Colbert's team says they have matched to codes used by Rackstraw’s Army unit in Vietnam. That letter’s writer—who Colbert believes to be Rackstraw—claimed he used a toupee and a putty nose to disguise his appearance on the plane.

Rackstraw was at one time considered a suspect by the FBI but was later cleared in 1979. After initially teasing that he might be the culprit, Rackstraw backed off those claims and insisted the accusation was without merit. The bureau officially closed the case in 2016, citing a lack of strong leads. In February 2018, Colbert claimed the FBI wasn’t acknowledging his work out of embarrassment.