Robert K. Ressler (1937-2013) was a supervisory special agent of the FBI as a reserve colonel in the U.S. Army's Criminal Investigation Division (CID) before retiring. He later served as the director of Forensic Behavioral Services, a business dedicated to training, learning, consulting, and expert witness testimony. He is the author of Whoever Fights Monsters and the inspiration for one of the main characters in the Netflix Original Series Mindhunter.