Colonel Frank Simmons was a member of the NID. His personnel file is classified, but what is known is that he was recruited by Harold Maybourne. In 2001, he became involved in several investigations concerning the Stargate Program, particularly SG-1. His access code was User 4574, which he used to access SGC personnel files. Later, he caputured a Goa'uld that took Adrian Conrad host without the knowledge of the NID, and was arrested as a consequence. He was eventually released during a hostage situation onboard the prototype Earth vessel, Prometheus, where he was taken host by Conrad's Goa'uld and killed by Jack O'Neill.
Because his personnel file is classified, much about his life prior to 2001 never came to surface. What is know is that he was born in the United States on 4th November, 1955. Before the year 2000, he was reputedly recruited into the NID by Colonel Maybourne. After Maybourne disappeared from the NID in 2000, following the exposure of his unauthorized off-world operations, Simmons took over many of his old duties within the NID. He would investigate several matters concerning the Stargate Program, particularly its flagship team, SG-1. He is pretty much despised by several personnel of Stargate Command, but unknown to them, the NID and the Pentagon, he had his own agendas.
He first arrived in Stargate Command in 2001, after SG-1 found a weapon on Velona, and Major Carter saw Orlin, believing that she was delusioned, he pushed the SGC to test the weapon, despite Carter's warnings that the Others would use their power to destroy the weapon, and the teams testing it, the teams went ahead anyway. After learning that Orlin was not a delusion, and was infact real, he lead a squad to capture him at Carter's residence, only to arrive and find he disappeared through his homemade Stargate to stop the weapon's test, which he succeeded in doing. (SG1: "Ascension")
Later, while SG-1 thought they had a fifth member, a Lt. Tyler, Simmons used his access code, User 4574 to gather information on SG-1 members, and used their histories for interrogation, which he claimed were relevent to his investigations (this included Dr. Jackson's relationship to Sarah Gardner, who became host to Osiris, and Teal'c's brainwashing by Apophis) When he interviewed Carter, she became aware that he was User 4574, Simmons threatened to destroy her career because she hacked the system during his investigation of her and her team. Afterwards, he accused Hammond that his personal feelings were influencing his duties, and warned him that "administrations change". He then left the SGC. (SG1: "The Fifth Man")
Months later, after Carter was kidnapped by Adrian Conrad, because he thought she held the answers on how to remove a Symbiote after implantation, oln him, Maybourne confronted O'Neill and claimed Simmons may have been responsible. After O'Neill's confrontation with him (after Simmons made him wait for two hours in the Pentagon), Simmons showed him evidence that he was in-fact responsible. (Maybourne was paid $3 million in his Cayman Island account for selling a Symbiote) After O'Neill and his team recovered Carter, he found Conrad, who was already taken host to a Goa'uld. Before O'Neill can capture him, Simmons appeared from behind and shot him twice in the arms with a silenced sidearm. (so no-one else could hear him) Simmons offered Conrad an escape. Simmons later visited him in a cell and tells Conrad's Goa'uld that he can offer it a lot in exchnage for information. He kept the Goa;uld prisoner without his superiors knowing about it. (SG1: "Desperate Measures")
After Teal'c was trapped in a wormhole after destroying Tanith, Simmons hired Rodney McKay to find a solution with Carter. Simmons pressed the SGC for a 48 hour deadline before the Stargate was to be used again, which would effectively kill Teal'c. Simmons later privately spoke with Hammond and told him that he would get Teal'c out in exchange for a kara kesh. Meanwhile, Maybourne and O'Neill tracked Simmons' movements and found the Goa'uld Simmons held. O'Neill informed the SGC, and Simmons was subsequently arrested and convicted for treason. (SG1: "48 Hours")
Several months later, Simmons and Conrad were released due to a hostage situation aboard the prototype USAF ship Prometheus, where the hostages were Carter and Jonas Quinn. After he was taken onboard, he worked with the hostage takers, and sent the ship was to hyperspace. After Carter shorted out those controls, Conrad said Simmons was no longer any use to them, so Simmons was forced to kill him. However, the Goa'uld then took Simmons as a host, though within moments of the medling, O'Neill opened the doors, and Simmons was sucked into space before the doors closed again, and died. (SG1: "Prometheus")