He was testing a gun that he'd converted from a normal pistol into an automatic weapon. He had been in the business of doing this for some time at the time of his death. Sarah Sidle was able to trace the weapon he had been using to him through its serial number. Ironically, however, the F.B.I. had assigned an undercover agent to track down John Doe and Sarah and Rick crashed their rendezvous four days after he died.