Terry Edward Mayhew was a radical and part of a civilian militia group.
Mayhew met Alex Krycek, who told him his name was Arntzen, during 1996. Krycek had some ideas for the group, including bombs and the Black Cancer, and they decided to act on them. Unbeknownst to him, Krycek was releasing information about them to Fox Mulder. In Queens, New York City, the group was raided and Mayhew was caught. He saw Krycek talking to Mulder. (TXF: "Tunguska")
Later, Mulder came to him in his cell to ask about Krycek. Mayhew told them what he knew, though stopped when Mulder asked about the second bomb. Mulder squeezed the information out of him and he told him about the truck in Terma, North Dakota. (TXF: "Terma")