Law enforcement generally tends to ignore everything else once they have a suspect they deem solid, and devote all their efforts to dredging up any sort of evidence against him they can. When you have the entire resources of the FBI crime lab behind you, and have already closed your mind to accepting or even contemplating any other perpetrator, it is depressingly easy to develop enough circumstantial evidence against almost anyone to proceed to charging him.
Evidence other than fingerprints, DNA, and ballistic or other toolmarks that can be directly tied to a suspect in an actual case is usually much more ambiguous in meaning than the idiots who think CSI is anything like real life can comprehend. Sadly there are people in the FBI and the rest of DOJ who really do believe that.