If they have been able with the recovered bullets to determine the caliber not hard and the make of the pistol used then they could go to the manufacture and ask for records of guns of that type sent to dealer's in that area.
no gov data base would be involved
Understand your thinking on the non-use of .gov database but it seems the legal logic breaks down when considering how they are able to get the manufacutrer's info when they have no legal authority over the out-of-state manufacturer. The only legal authority the manufacturers are likely to recognize is BATFE and they require info on a specific gun in order to initiate a trace request, not a blanket fishing expedition.