If they found the gun used in the shooting (was it left at the scene?) then they could run the serial number through the NICS database (you know, the registry of guns and gun owners that the 1986 FOPA made illegal but you know they maintain) then you could find the last person to purchase the gun from a dealer.

So basically when they find these guns they can find the last LAW ABIDING CITIZEN that owned the gun and they can crucify him (you know that makes them happier than finding a genuine criminal)