Anyone that knows how pistols operate knows it wasn’t. Why is there so much speculation it was an exotic weapon?

Only the other day the news was that there were casings with words etched on it and a unfired round from clearing the jam. Surely, those casings would tell them what sort of gun was used.

Not really, it just tells you the calibre of the pistol, and maybe some details on the loading mechanism of you're lucky enough to get distinctive marks on the casing.