some sort of tracking of him was done that the feds don't want people to know about
That "tracking" is all the work they did to find a suitable fake perp. If they had any real evidence and Luigi did do it, it would've been admitted almost immediately. They have very little to lose.
it was something that may be ruled unconstitutional and risk the case against him
And besides, if that were true and Luigi truly did it, don't you think the current SCOTUS would use this great stroke of luck as a way of undoing some "dangerous" precedent?