reporter: this is a ghost gun. it took no background check to purchase it because it s built with a perfectly legal prepackaged kit and then made into a working firearm, in just over an hour. it was a black market ghost gun sale that went bad and turned deadly. the 16-year-old student gunned down two classmates law enforcement tell nbc news the weapon may have been a ghost gun. the firearm in evidence is described as an 80 percenter, a ghos gun. reporter: in philadelphia, the police department last year recovered 571 constructed ghost guns more than five times the number in 2019 40 states have not regulated the selling of these kits, and that s why we were able to walk into this gun show outside philadelphia we went in with a hidden camera to show just how easy it is to buy a kit. you don t need anything from me, right? money. money but no i.d. or anything we re not doing background checks? no. have you seen the demand for your supply going up yeah.