The Behavioral Science Unit (also known as the Behavioral Sciences Unit) was a department of the Federal Bureau of Investigation. (TXF: "Tooms", et al.) It was based in the FBI Academy, in Quantico, Virginia. (MM: "Lamentation", "Seven and One") One of the activities that took place in the Behavioral Science Unit was the profiling of serial killers. (TXF: "Tooms") Sometime prior to 1998, the Critical Incident Response Group originated within the Behavioral Science Unit and at first shared its name. (MM: "The Innocents")

For a period of three years that included 1990, Special Agent Fox Mulder worked at the FBI's Behavioral Sciences Unit, profiling serial killers. (TXF: "Tooms", "Travelers") In 1990, he introduced himself to former Special Agent Arthur Dales as a profiler for the unit. (TXF: "Travelers") Mulder had left the unit by 1994 and, in that year, he announced his duration of work for the unit, when asked to state his qualifications upon attending the trial of Eugene Victor Tooms as an expert witness. (TXF: "Tooms")

Following the closure of the X-Files later that year, Dana Scully, Mulder's former partner, suggested to him, while he was considering quitting the FBI, that he could request a transfer to Quantico, where Scully was currently assigned, and return to the Behavioral Sciences Unit. Mulder initially seemed reluctant to follow Scully's advice, arguing that their FBI superiors did not want the two agents working together. (TXF: "The Host")

Shortly thereafter, Sheriff Spencer, who had asked for Mulder's help on a particular murder investigation in which all the suspects were dead, remarked to Mulder about the strangeness of the case, stating that he realized the FBI's Behavioral Science Unit normally profiled murder suspects that were still at large and indirectly referring to Mulder as a representative of the unit. (TXF: "Blood")

Later the same year, Agent Scully mentioned that extrapolating from material on file at the FBI's Behavioral Science Unit could give the clearest possible idea of a death fetishist's compulsion. (TXF: "Irresistible")

In 1997, a class of FBI agents, under the leadership of Special Agent Tom Babich, was organized in the Behavioral Sciences Unit in Quantico, where Babich introduced them to Frank Black, acknowledging that the class was more than likely familiar with Frank's work with the same unit that they themselves were now part of. Together, these agents searched for escapee Dr. Ephraim Fabricant, whom Frank had previously profiled. (MM: "Lamentation")

After Special Agent Emma Hollis introduced herself to Frank Black as a member of the Critical Incident Response Group in 1998, she commented to him that she thought he had been at the Bureau when the group had all still been called the Behavioral Science Unit, a belief that he then confirmed. (MM: "The Innocents")

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