I read somewhere that Larry Carr volunteered to be the Cooper case agent, and was expecting to walk in, look at the evidence and prove McCoy was Cooper.
Not only did Carr not do that, he changed his mind to the point where he believes Cooper wasn't any of the known suspects and that Cooper must have died in the jump.
If you read the files, you realize just how much effort was put into investigating McCoy with a null result. McCoy was home at 10am on Thanksgiving day and was probably in Salt Lake city the day of the hijacking. None of the witnesses thought it was McCoy and he didn't fit the description or the FBI profile. The particles on the tie completely eliminate him.