Armin Selig was a reporter who, somehow, learned about the Stargate Program (most likely through a leak).
He approached Colonel Jack O'Neill, while he was at a bar in Washington D.C. after attending a ceremony in the city. He revealed his knowledge of the Stargate program's existence, and supported his claims with a recording of a conversation O'Neill had with Captain Samantha Carter discussing the Stargate program, while they were walking through a public square, that he apparently obtained with sophisticated listening devices, and asked for a comment on the matter. Although O'Neill firmly denied any knowledge, he promptly warned Major General George S. Hammond of this evidence of a security leak.
Hammond ordered him to speak to Selig again, to ascertain how much he knew, and O'Neill was to recontact him by simply loitering on the street and Selig approached him. Unfortunately, the resulting conversation revealed that Selig knew a considerable number of details and was sure enough, as to their veracity, to intend to print them shortly, despite O'Neill's firm and consistent denials.
Immediately after this confrontation, Selig was fatally wounded in a hit-and-run. Although O'Neill immediately called for an ambulance and attempted to render assistance, Selig, with his dying breath, accused O'Neill of arranging the attack to silence him. Although O'Neill did no such thing, the fact that this apparent accident so conveniently solved the Stargate Program's secrecy problem led him to suspect that the NID murdered Selig, a suspicion that Hammond firmly denied. (SG1: "Secrets")
Joe Spencer, who had inadvertently gained access to O'Neill's memories through a Long-range communication stone, states that Selig was, in fact, killed by the NID. However, it is likely he was merely voicing O'Neill's own suspicions, as he would be in no position to know any better. (SG1: "Citizen Joe")