Monroe tells me that the selection process was brutal: there were physical and psychological tests, interviews and 16 weeks of training at the FBI Academy. And she still remembers her first day on the job, assigned to a criminal squad in New Mexico – including being nearly two hours early to the office and having to sit outside and wait for someone to let her in. “I was incredibly eager,” she says, explaining that, unlike the office-based work of her police career, her first day even involved making an arrest.

“I may have been overwhelmed at times, but I was certainly never bored,” says Monroe, when I ask about how life in the FBI continued. She and her colleagues used to joke that the F in ‘FBI’ is for flexibility, as whatever you may have put in your calendar the night before has probably changed to something completely different when you get to the office the next morning. “You were always being pulled in a thousand directions: a bank robbery over there and the US attorney flagging something to your team over here.”

Monroe was promoted to the Behavioural Science Unit (BSU) in 1990, studying the perpetrators of countless high-profile crimes including serial killers such as Ted Bundy. “He’s frightening to me in a very psychological way; the horrific things he did were truly diabolical. When I think about how much studying has gone into this kind of behaviour and the ‘why’ behind it, I don’t think we’ve ever come close to an answer,” Monroe says. As an agent, Monroe was never in the same room as Bundy (he died a year before she joined the BSU) but she has interviewed many other offenders.

Monroe spoke to serial killer Edmund Kemper – who killed eight people, including his mother – over the phone when he called to offer his help on another case she was working on. “Kemper spoke in the same flat monotone he uses in prison interviews readily available on YouTube, but there was something about the implied intimacy of our telephonic tete-a-tete that left me frankly shaken when it was over,” Monroe writes in her book. She was also supposed to interview Jeffrey Dahmer in prison but two days before it was due to take place, he was killed by a fellow inmate in November 1994.

“When I’m trying to elicit information from someone that’s done something horrific, you can’t go in all judgmental. Because when that person starts to feel a level of ease or comfort with you, that’s when they’d tell us information. The ego is an amazing thing.” But she adds, she was always glad she could “never understand their behaviours”.

Munroe always took handwritten notes during her time in the FBI, which was a huge help when it came to writing Hearts Of Darkness. “My house looked as though I was moving, there were boxes of diaries and notebooks everywhere,” she says.

In going through all these boxes, Monroe has had a lot of time to reflect on the impact of her immersion in such shocking and violent cases. “I had worked on a lot of cases where the victims had been stabbed with their own knives by intruders in their homes. And so, I thought, if someone’s going to break into my home, they’ll have to bring their own knife and I took the knives in our house and put them in the dryer in the basement.”

“I hid my knives in the dryer in the basement”: the secrets of a former FBI agent