In terms of evidence, Iam sure we can both agree that the U.S. government has a very long and documented track record of coverups and the withholding of information from the general public concerning a wide range of issues.
And when pressed on these issues, representatives from the government will reply that in the interest of national security, this will remain classified.
So based on these facts, it is reasonable to suggest that the truth about the D B. Cooper case has been classified by the FBI, and is not accessible through the freedom of information act.
I also believe that the Jon Benet Ramsey case has also been solved, but that the findings remain classified to this day.