of them threw the pieces of the gun off this pier. an fbi dive team out of miami was soon on the scene. the water was gin clear that day, and divers found forensic sunken treasure. there on the bottom, rusted, covered with seaweed and entrusted with barnacles, were the pieces of a handgun. the parts were rushed to the fbi crime lab in quantico, virginia. "dateline" was given rare access to the fbi's world-renowned crime lab. in his first ever interview firearms expert brett mills told us how he examined and reconstructed the weapon. first, he carefully cleaned the frame of the gun. >> when i finished cleaning it, there was no serial number there. from our reference collection i knew where the serial number should be. >> it was gone because delgado's ex-boyfriend had ground it away, totally obliterated it. or so he thought. mills had a plan.