Artist sketches of D.B. Cooper. (Images: Courtesy of FBI)

It's the case that won't go away.

The only unsolved case of skyjacking in U.S. history remains unsolved, but a team of investigators is confident it has identified the culprit, his co-conspirators, and new evidence covered up by the FBI for 46 years.

A 40-member cold-case team led by filmmaker and author Tom Colbert has pursued the case for several years, and after a recent suit to obtain public records, Colbert's team got its hands on a letter allegedly sent by Cooper to The Seattle Times and other newspapers in the weeks following the incident.

The letter confirms the team's suspect is correct and that the FBI has worked to hide the evidence for decades, Colbert told SeattlePI: "This letter proves to me that there's been a cover-up and continues to be a cover-up on this case."

Colbert's team, comprised of former law enforcement officers and several former FBI agents, previously identified Robert W. Rackstraw, a 74-year-old Vietnam veteran now living in San Diego, as the man they believe to be Cooper. Rackstraw was a suspect for the hijacking in the late 1970s, but formal charges were never pursued.

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