The life Dennis Rader lived on the outside hid his dark secret, which he was so desperate to reveal by dropping breadcrumbs to the media.

No one knows how long it had been hidden there. A seemingly innocent piece of paper caught between pages of an engineering textbook at the Wichita Public Library. But the words typed on there were the start of a cat-and-mouse hunt of the self-dubbed BTK Killer, Dennis Rader.

On the surface, Rader led a fairly typical life. He served in the Air Force in the 1960s and eventually married and settled down in Wichita and had two kids. He worked for the camping equipment company Coleman Company, home security company ADT and then as a Park City, Kansas, compliance officer. And to really hammer in the family man image, he was also an active member of his church and a Boy Scout leader.

But that picture-perfect facade may have been the exact image he wanted to relay as he covered up some of the most gruesome murders in American history.

The possible reason: As a child, he had developed “violent sexual fantasies that involved bondage” after killing animals. And on January 15, 1974, he turned to his first murder spree, killing the parents and two kids of the Otero family and then following up by murdering Kathryn Bright in April of that year. He had known both Bright and the Otero mother from his time working at Coleman Company.

In a cruel twist of timing, it was four years later that Rader graduated from Wichita State University studying — what else? — criminal justice.

And it was that kind of double life that led to his carefully orchestrated crime spree, which lasted from 1974 to 1991.

PHOTOS: Mugshots of Famous Serial Killers

BTK stands for 'bind them, torture them, kill them'

Adding to the horror of Radar’s murders was his constant toying with authorities, starting with that note left in a library book.

Wichita Eagle newspaper employee Don Granger received a phone call in 1974 revealing that the letter was stashed in one of the books. Granger immediately let officials know and the police found it. Yet the contents of the letter weren’t revealed until a new weekly newspaper, the Wichita Sun, which had only launched a few months before that, got their hands on the letter.

A portion of the letter said, "I can't stop it so the monster goes on and hurts me as well as society. ... It's a big complicated game my friend the monster play, putting victims number down, follow them, checking up on them, waiting in the dark, waiting, waiting…”

And in a postscript, it read, "P.S. Since sex criminals do not change their M.O. or by nature cannot do so, I will not change mine. The code word for me will be… Bind them, torture them, kill them, B.T.K., you see he's at it again.”

And thus the monstrous Rader gave himself the title he’s known as: BTK Killer.

Dennis Rader mugshot

Dennis Rader, aka the BTK Killer's, mugshot

Thirty years after the Otero murders, Rader began dropping hints again

About four years later, on January 31, 1978, the Wichita Eagle received another note, this time in the form of a poem starting with the words, “Shirleylocks, shirleylocks," on an index card about the murder of Shirley Vian, killed the previous March. Around the same time, the Eagle got another letter about the Otero murders and the TV station KAKE got a letter referring to the killings of Vian and Nancy Fox, slain in December of 1977, as well as another unnamed victim.

He reportedly drew pleasure from the media coverage, even expressing in one of his letters: “How many people do I have to kill before I get a name in the paper or some national attention?”

His last recorded murder was in 1991, but it was around the time of the 30th anniversary of the Otero family murders that Rader started to drop hints once again.

A KAKE viewer reported a suspicious box in December 2004, which contained a Barbie mimicking the murder of the one of the Oteros, as well as Fox’s driver’s license. A month later, the station got a postcard leading them to a cereal box with a note, "Can I communicate with Floppy [disk] and not be traced to a computer. Be honest."

While the disk did end up being relayed, Rader’s inability to hide the metadata from the documents led to his eventual arrest in 2005.

He was given 10 life sentences and remains at El Dorado Correctional Facility, with his earliest parole being set for the year 2180. His heartless spree sparked Stephen King’s A Good Marriage novella, as well as numerous documentaries. The character ADT Man on Mindhunter is also based on Rader.