I don’t know how these things work, but 20 years without a trial is criminal. Try the guy, then execute if convicted. If you don’t have enough evidence for a trial after this long perhaps he isn’t the guy? This sounds ridiculous.
I believe that one or two have been let go and ended up fighting against us in Afghanistan and involved in other bad behavior.
It's a no win situation. Being locked up so long they probably hate us. Even if by some chance they were innocent, they would probably spend the rest of their lives attacking us.