It won't be the first time Super Tucanos have operated in the US. XE Aviation (formerly Blackwater) has flown N314TG (photo) and displayed it at the 2011 Reno Air Races in Nevada. XE Aviation also has a Pilatus PC-6 registered under N181DA (photo).
Super Tucanos have also been photographed under military registrations, although nobody seems certain whether these are different airframes, or simply N314TG with different markings. Indeed, a thread on Scramble points out that the military registration seems to be a re-purposed MH-53E registration, perhaps to keep the aircraft (and possibly the "Imminent Fury" program) under wraps:
Another photo on Flickr shows the same airframe equipped with what appear to be guns or rocket pods underwing.