>> he goes it was. >> now please tell you have that weapon? >> he goes, i do. >> reporter: bingo. >> we got to get that gun. >> reporter: yeah. >> art went down and picked up the gun. we submitted it to the firearms tech. >> reporter: the report came back, clear as day, this was the gun that killed susan sutton and blinded her husband. which obviously connects garrett kopp to that murder pretty intimately. >> absolutely. >> reporter: but detectives did not rush out and arrest kopp. for a simple but very important reason. there was a bigger question that needed to be answered. did his friend ch...