The Third Man was a mysterious man who was present in FBI Headquarters in March 1992.

On March 6, the Third Man was in Section Chief Scott Blevins' office with the room's usual occupant and the Cigarette Smoking Man, during a meeting with Special Agent Dana Scully. After Scully had arrived and answered a question Blevins posed to her regarding her career history, the Third Man asked if she was familiar with an agent named Fox Mulder, to which she replied positively. The Third Man then asked how she knew Mulder or knew of him, prompting her to reveal that she was familiar with Mulder by reputation and to divulge the specific details she knew of him. The Third Man continued to witness the meeting as Blevins assigned Scully to work with Mulder on the X-files and write field reports regarding their investigations.

After the agents returned from their first case together in Oregon, the Third Man, along with the three other individuals who had been present at the earlier meeting, secretly watched from behind a mirrored window while teenager Billy Miles underwent hypnosis, recalling his role in a series of recent abductions, but the men soon left the room, followed by Agent Scully.

The Third Man returned to Section Chief Blevins' office with Scully and Blevins himself, but without the Cigarette Smoking Man. As Blevins then questioned Scully in the office, the Third Man implied that he believed the majority of phenomena she had described in her field report were things she had not actually seen for herself and asked her to clarify whether she had or had not experienced time loss described in her report, but she answered that she could not substantiate the phenomenon. The Third Man witnessed the entirety of this meeting and - when Scully presented an implant, the final piece of existing physical evidence from the case, to him and Blevins - he thoroughly looked it over before Scully left the room and the Cigarette Smoking Man returned there, subsequently taking the implant to a storage room in the Pentagon. (TXF: "Pilot")

The Third Man went to Skinner's office to interrogate Scully on what she knew about Mulder and his possession of sensitive data. (TXF: "Anasazi")

The Third Man heard Scully's explanation in Skinner's office, along with Skinner and some others, about Jeremiah Smith's records being smallpox records and showed skepticism. (TXF: "Herrenvolk")

The Third Man was present with Blevins when Scully came for questioning about Mulder's apparent death. He asked Scully about Kritschgau and the hoax. (TXF: "Redux") He then assisted Blevins with questioning Mulder when he came back. At the FBI hearing about Mulder's involvement in the murder of Scott Ostlehoff, he watched as Mulder refused to answer until he made an accusation about who was behind the spying on his apartment. He pointed to Blevins as being behind the whole thing, and so the Third Man acted. When Blevins fled to his office, the man was waiting for him, got up and shot Blevins as soon as he walked in the door. He left the gun as if Blevins had killed himself and was never heard from again. (TXF: "Redux II")

The Third Man was played by Ken Camroux.
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