In 1999, Detweiler tracked a wanshang dhole in China. Instead of catching it, Detweiler was somehow transformed into one. While in the form, he was "captured" and brought to the US via the T'ien Kou. When two curious crewmembers opened the case during the trip, he killed both of them and locked them back in the container. On arrival, he attempted to hide his condition from investigating agents and FBI agents Fox Mulder and Dana Scully but would morph into the wolf-like being each night and viciously kill more people, including Jake Conroy and Frank Fiedler.
At the scene of Fiedler's death, Detweiler met Karin Berquist and was instantly uncomfortable with her, though tried to ignore her when talking to Mulder. He suspected that Berquist could sense he was the actual killer.
Detweiler overheard Jeffrey Cahn say he would kill the animal instead of kill him, and Detweiler threatened him if he did so. Detweiler then obtained tranquilizer drugs from Dr. James Riley, but went back to the hospital afterwards and killed him by pretending to be another dog, getting shot by Cahn when Riley called in his presence, and then attacking Riley after he patched him up.
Detweiler went to kill Cahn as well, though that time he did not finish the job. Rather, he waited by Cahn's bed at the hospital. He was challenged by Mulder, who by then realized the truth. Detweiler feigned ignorance and left, only to be told by Berquist, right outside, that she would not protect him any longer. He decided to kill Berquist later, however, when attacking her in her home, they both fell out the second-story window and Detweiler was impaled on a post below. (TXF: "Alpha")