The BAU is called to St. Louis to assist the investigation into two independent serial killers who appear to be competing with each other. Meanwhile, Gideon and Hotch are visited by a prospective new team member, Emily Prentiss.
- Hanna Alltop - Hysterical Woman #1
- Sara Alltop - Katie Carroll
- Karina Bennett - Female Reporter #2
- Christopher B. Duncan - Agent James Sheridan
- Bethany Geaber - Hysterical Woman #2
- Kate Rene Gleason - Female FBI Agent
- Landall Goolsby - Male Reporter #2
- David Barry Gray - Tom Carroll
- Jeff Grays - Sheriff's Deputy
- Rochelle Greenwood - Marci Mitchell
- Chip Hormess - Jason Mitchell
- Jonathan Kehoe - Angry Man
- Victoria Kelleher - Ellen Carroll
- Robert C. Koch - ND Agent
- Scott Michael Morgan - The Hollow Man
- Trevor Olsen - Male Reporter #1
- Jason O'Mara - The Mill Creek Killer
- Natalie Salins - Cara Becker
- Maury Sterling - Jim Meyers
- Inger Tudor - Female Reporter #1
- Cory Tyler - Security Guard
- Deborah Van Valkenburgh - Betty Mitchell
- David Berkowitz, Son of Sam
- Paul Bateson or Richard Cottingham, The Torso Killer (possibly; unidentified)
- Russell Ellwood and Victor Gant (possibly; unidentified)
- Dale Hausner and Samuel Dieteman, The Serial Shooters
- Mark Goudeau, The Baseline Killer
- Ted Bundy
- Mark David Chapman
- "Field Below" by Regina Spektor
- Jason Gideon: Elbert Hubbard once wrote, "If men could only know each other, they would neither idolize nor hate."
- Aaron Hotchner: Mahatma Gandhi once said, "Remember that all through history there have been tyrants and murderers, and for a time they seemed invincible. But in the end, they always fall. Always."
- First appearance of Paget Brewster in the main cast as SSA Emily Prentiss
- Though Reid only names three examples of instances when more than one independent serial killer has been active in the same area at the same time, there are several more examples, including (but not limited to):
- Edmund Kemper and Herbert Mullin were both active in Santa Cruz, California, at the same time.
- Patrick Kearney, William Bonin, and Randy Kraft were three independent serial killers in California at the same time, whose individual strings of killings were initially believed to be committed by a single killer nicknamed "The Freeway Killer."
- Derrick Todd Lee, a.k.a. "The Baton Rouge Serial Killer", was active in Baton Rouge, Louisiana, at the same time as Sean Vincent Gillis, who actually feared being "outdone" by Lee.
- Lonnie David Franklin, a.k.a. "The Grim Sleeper," was initially believed to be an unrelated serial killer referred to as "The Southside Slayer," who turned out to be at least five independent killers: Louis Craine (who committed at least two of the Slayer's murders), Michael Hughes, Daniel Lee Siebert, Chester Turner, and Ivan Hill (the latter four of whom were attributed to at least one Slayer murder each).
- In addition, an unrelated series of murders, which was erroneously dubbed the "Strawberry murders", began in succession of the murder of Debra Jackson, a victim of the Sleeper; these murders had a similar M.O. to Jackson's murder.
- Joel Rifkin and Robert Shulman were both active in the Suffolk and Nassau Counties of Long Island, New York, at the same time; they even lived a few miles away from each other and killed some of their victims on the same days.
- After Rifkin's arrest in 1993, Shulman then shared the same area of operation with an unidentified third serial killer known as (among other names) the Gilgo Beach Killer until his own arrest in 1996.
- Anthony Sowell and Joseph Harwell were both active in Cleveland, Ohio, at the same time.
- Moses Sithole was active in Johannesburg, South Africa at the same as David Selepe.
- Cedric Maake also became active in Johannesburg while Sithole was awaiting trial for his own murders, prompting demands for the reestablishment of the then-recently abolished death penalty in South Africa, which went unheeded. Maake's M.O. was so varied that he was wrongly believed to be two different killers at the time ("The Wemmer Pan Killer" and "The Hammer Killer.")
- "The Cottonfield Female Killers" and "The Rebels of Ciudad Juárez" were two separate killing teams that raped and murdered women in Ciudad Juárez, Mexico at the same time.
- Juana Barraza, Araceli Vázquez, and Jorge Mario Tablas were active in Mexico City at the same time. However, all three could have been motivated in the first place by wrongful reports of a serial killer active in the city.
- London, Ontario, Canada, was at one point infamous for having an alarmingly high concentration of serial killers active at the same time. Between 1959 and 1984, 32 murders of women and children occurred; thirteen were attributed to three different killers: Gerald Thomas Archer, a.k.a. "The London Chamber Maid Slayer"; Christian McGee, a.k.a. "The Mad Slasher;" and Russell Johnson, a.k.a. "The Balcony Killer." The remaining nineteen murders (which remain unsolved) are believed to have been the work of as many as three other serial killers, according to an independent investigation by Dennis Alsop, a detective sergeant with the Ontario provincial police; and Murder City: The Untold Story of Canada's Serial Killer Capital, a book written by criminology professor Michael Arntfield.
- Albert Fish was active in New York City at the same time as another serial killer of children, Peter Kudzinowski (who was also active in New Jersey). Kudzinowski was the main suspect in the disappearance of four-year-old Billy Gaffney before Fish confessed to his murder.
- Jeffrey Dahmer and Ottis Toole were both in South Florida around the time of Adam Walsh's abduction and murder from a Hollywood, FL mall in 1981, leading to speculation that either was responsible.
- ↑ Referred to as a serial killer active at the same time as David Berkowitz
- ↑ Referred to as two serial killers active at the same time in New Orleans