Frank Nicholson was an FBI Agent who worked in the Washington D.C Office.
Nicholson was part of the task force under Edward Hamshaw, and received the tip off from his informant Shevy Dominguez that a terrorist cell were buying weapons at Cody's Storage Warehouse on the waterfront. On this tip, the taskforce seized the warehouse and five agents, including Hamshaw, were ambushed and murdered by Dario Vukjovic. Hamshaw took command of the task force, who remained on station at the warehouse but failed to find the gunman. He was relieved by Head Office, and gave an account of the incident at a briefing the following morning.
After asking for a meeting with Deputy Director Skinner, Nicholson lambasted Agent Fox Mulder's presence on the case and accused Skinner of bias. After Skinner attempted to calm him down, Nicholson exclaimed that it "wasn't over", before storming out.
After becoming remorseful about his actions, Nicholson apologised to Skinner and was released on suspension. He later visited Mulder at his apartment, apologising for the attack and admitting the guilt he felt for the deaths. Mulder convinced him to accompany him as back up to Scully's apartment, and he complied. Mulder explained that the gunman was Vukjovic, and that the whole situation was engineered as a diversion. After arriving at the apartment, Nicholson went round the back of the building and found Scully restrained and unconcious. Before he could free her, he was killed by Vukjovic. ("Dead in the Morning")