it is not hard to legally buy a handgun. however, only tennessee residents can buy handguns from a gun dealer in TN. you must pass a background check, and must obtain a permit to carry a gun concealed on yourself or in your car. there is no law in tennessee that requires citizens to register guns when they buy/sell a firearm to another citizen. these guys could have bought this gun off an individual or at a gun show and it be completely legal (as long as they are 21 yr old, which i think they all are).
as far as i can tell from the reports i have heard, the only law these guys broke was not having a permit for these guns and the altered serial number. which i am still puzzled about; i thought altering a serial number on a firearm was a felony.
But im sure ATF will conduct a gun trace and find out how they obtained these firearms.