The Netflix series Mindhunter follows two FBI agents in the 1970s and 1980s as they attempt to amass profiles of serial killers. These agents, Holden Ford and Bill Tench, travel the country, interviewing famous sequential slayers.

The show is inspired by the book Mindhunter: Inside the FBI’s Elite Serial Crime Unit, written by retired FBI agents John E. Douglas and Mark Olshaker. Like Ford and Tench in the show, Douglas and Olshaker spent decades interviewing and profiling serial killers. Douglas co-authored another book with the FBI agent who coined the term “serial killer,” Robert Ressler, titled Sexual Homicide: Patterns and Motives.

Mindhunter’s writers have relied on dozens of other books about the FBI and serial killers in order to make their show about the early days of psychological profiling as realistic as possible. Here are 10 things in the series that only make sense to those who have read the source material.

Son Of Sam’s Interview About The BTK Killer

David Berkowitz, known as the Son of Sam, was a real-life serial killer who terrorized New York City during the summer of 1976. After killing six people, he was apprehended by police, claiming to have been compelled to act at the behest of his neighbor’s dog, which was possessed by a demon. John E. Douglas interviewed Berkowitz, and what he learned about the murderer’s motivations and delusions informed his books.

In Mindhunter, Berkowitz is interviewed by Ford and Tench because they are investigating the BTK case in Kansas. Dennis Rader, the BTK killer, was never interviewed by Douglas or Olshaker. One reason for this is that Rader wasn’t caught until 2005. In the fictional world of the show, though, Ford and Tench hope chatting with Berkowitz will help them gain some insight into BTK’s psyche. Both BTK and Berkowitz wrote letters to the media, and this line of similarity is the sort of pattern profilers look for.

Ed Kemper Cornering Agent Ford In His Hospital Room

In the first season, the most notorious and terrifying serial killer interviewed by Ford and Tench is Ed Kemper, known as the Co-Ed Killer. After taking the lives of his grandparents as a teenager, he killed eight people after being released from a mental institution at the age of 21. He was caught in 1973. In the first season’s finale, Ford questions Kemper while the lumbering man with a distinctive voice and gesticulations corners and hugs the agent.

While this scene is cringe-worthy and dramatic, it wasn’t placed in the show only for added suspense. This close-call happened to Agent Robert Ressler, although not in a hospital room. Kemper got a little too close for comfort once in his jail cell, claiming afterward it was just a joke.

Ford Staging The Interrogation Room When He Questioned Darrell Gene Devier

This tense interrogation scene, which Agent Ford stages with photos and evidence from the murder of a 12-year-old girl, is based on Douglas’s writing. As an FBI agent, Douglas used the opportunity to interrogate Devier as a way to test out his theories about how to get genuine confessions.

Devier was a serial killer and tree trimmer who was questioned after a girl’s body was found, and Ford manipulates Devier by using disgusting and misogynistic language to make the man think he’s on his side. Ford indicates the girl “was asking for it,” and filling the room with evidence from the crime scene eventually forces a confession out Devier.

Why Richard Speck Throws A Sparrow Into A Ceiling Fan

Richard Speck tortured and murdered eight student nurses on the night of July 13, 1966. He was interviewed by Douglas and Ressler, and both wrote about him extensively. In Mindhunter, when Ford and Tench interview the remorseless killer, Speck tosses a sparrow he’d nursed back to health into a ceiling fan,which is an alarming and horrific scene.

This scene isn’t fabricated just to add a new layer of dread to the show’s storytelling; Richard Speck really did do this to a sparrow. Douglas and Ressler didn’t witness this firsthand, though, but were told about the incident by a security guard at the Illinois prison where Speck served his life sentence.

Brian Tench’s Strange Behavior

One of the most confounding plotlines in Mindhunter involves Agent Tench’s adopted son, Brian. While the boy struggles with emotional and psychological development in the first season, his progress is even more stunted after he witnesses other boys in his neighborhood kill a toddler in the second season.

The incident is based on a true incident, but none of the agents whose books were used to make the show write about familial issues like this. Without the books, though, the psychological underpinnings that affect Brian’s growth and ability to relate to others wouldn’t make sense.

The Surge In “Sequence” Killers During The 1970s and 1980s

Mindhunter traces a heinous trend that defined the 1970s and 1980s: a surge in serial killings. The show itself highlights the murderers who made these decades bloody and dismal. Without knowing the source material for Mindhunter, it’s easy to overlook this basic premise that makes the show’s storytelling capacity so expansive.

While it’s hard to understand exactly why the 1970s and 1980s experienced such an excess of serial killers, theories abound, many of which are hinted at or directly explored by Douglas, Olshaker, and Ressler in their books. Law enforcement was not as connected and organized then as it is now. In order to solve multiple crimes by one killer, investigators needed a lot more data than they had access to, meaning it was easier to commit odious crimes and get away with them.

Why Season Two Focuses On The Atlanta Child Murders

The second season’s arc involving the murders of dozens of black children in Atlanta between 1979 and 1981 is one of the most harrowing on Mindhunter to date. As Ford and Tench narrow in on suspect Wayne Williams, the racism and police corruption that led to the deaths of an estimated 30 children are exposed.

Douglas and Olshaker interviewed Wayne Williams for their books. While Williams was never officially charged for the murders of any children, he was indicted for the deaths of two black men in 1981, which corresponds to the time the child murders stopped. Douglas does not believe Williams is responsible for all the deaths, and numerous attempts have to been made to reopen the cases or create some kind of legal traction.

Charles Manson Sitting On Top Of His Chair During His Meeting With Agent Ford

When Agent Ford interviews Charles Manson, it’s not just an attempt on the part of Mindhunter writers to appeal to viewers’ knowledge of serial killers; Douglas actually interviewed Manson extensively and describes his interactions with the infamous cult leader in his Mindhunter book.

Details from the book were used to frame Ford’s interaction with Manson, who he almost seems to revere. Douglas writes about Manson sitting on the top of his chair in an effort to maintain control over his interviewer.

Dr. Wendy Carr’s Portrayal As A Queer Woman

Wendy Carr’s character is also based on a real-life FBI investigator alongside Douglas and Ressler: Dr. Ann Wolbert Burgess. She worked with them on the book Sexual Homicide: Patterns and Motives after serving with them in the FBI’s Behavioral Sciences Unit.

While nothing else is known about Carr’s inspiration, the show portrays her as a queer woman who has learned to separate her personal and private lives, at times to a fault. As a female character in a show about serial killing men, Carr’s queerness subverts the corrosive and toxic masculinity that exists everywhere around her.

References To John Wayne Gacy In Season Two

Season two includes a few references to John Wayne Gacy, which many fans have interpreted as signs he will play a more prominent role in the next season. Whether or not this is true, Douglas spent time with the serial killing clown, and his writing about Gacy was used for the show.

Gacy was convicted of 33 murders in 1980 after dozens of men’s bodies were found on his property. Douglas claims Gacy continued to call him at his office for years after their meeting.