In the letter, the author states that he left no fingerprints on the airplane. This is critical towards authentication is because it is 100% accurate.
The FBI got its best lead in 1980 when a young kid found a bundle of rotten $20 bills near the Columbia River, in Washington. The serial numbers on those bills matched the ones stolen by Cooper. You can now even purchase some of these bills for your private collection.
Colbert and his team of 40 volunteer members of law enforcement believe that Cooper was really Robert Rackstraw, a 73-year-old man living around San Diego, though this has never been proven. Rackstraw has even sent letters to the court overseeing the Freedom of Information Act proceedings, asking the judge to stop this charade as Colbert's accusations have ruined his life.
We will likely never know the true identity of the infamous D.B. Cooper.