Shortly after the shooter was confirmed dead an AFT employee appeared on the roof and took cell phone photos of the SN of the rifle used. The ATF then looked up in their illegal database the name and address of the purchaser of the rifle. The legal method for the ATF to trace a firearm is by contacting the manufacture for the name and address of the FLL that the gun was sold too then going to that FLL to view the ATF Form 4473 filled out by the person who purchased the firearm from the FLL