The history channel had a recent program suggesting that it was a disgruntled employee that did it. Relative or friend came forward suggesting it. Guy was ex military, etc.
...an interesting expose on DB Cooper on a show called “Decoded” on Discovery channel about 6 months ago
They interviewed (who the believe to be) DB’s brother here in MN.
http://www.youtube.com/watch?v=v66OsQR0Zow
he had means, proximity and intent.