Why are some people so against transferring the Gitmo prisoners to US soil?

As if they have no confidence in our own prison system? We held Timothy McVeigh and all his ociates behind bars just fine as I recall, and they carried out the deadliest terrorist attack on US soil prior to 9-11. We keep serial killer, rapists, and child molesters in check every day, but we can't handle some prisoners who may or may not even be guilty?