 While traveling around the country providing instruction to police, Douglas began interviewing serial killers and other violent sex offenders at various prisons.He interviewed some of the most notable violent criminals in recent history as part of the study, including David Berkowitz, Ted Bundy, John Wayne Gacy, Charles Manson, Lynette Fromme, Sara Jane Moore, Edmund Kemper, James Earl Ray, Sirhan Sirhan, Richard Speck, Donald Harvey, Gary Ridgway and Joseph Paul Franklin.He used the information gleaned from these interviews in the book "Sexual Homicide