How much detail has been released about passengers, crew members, and airline/airport personnel? Is every one of the passengers accounted for? Pictures? Names verified, addresses verified? Some investigators assumed he had help, maybe even from an employee. However, if they never found DB Cooper, then they don't know how to find similarities between him and others (related, worked together, served together, lived near each other).
I'm not trying to start a conspiracy discussion, but it is standard practice to vet out all witnesses and relatives. Am I safe in assuming it was done? The descriptions were all over the place, the composites were all over the place. Maybe he didn't act alone. For the record, I definitely do not think Flo, Bill Mitchell, or Ratazack were involved.