Warning: This article has graphic content that may be disturbing to some viewers.

In one of the BTK Killer's several letters to police, serial killer Dennis Rader shared disturbing details from his childhood that explained the motivations behind his brutal, sexually charged murders. Rader explained that as a boy, he would often view pornography, which he believed led him to develop violent fantasies as a teenager that involved "S/M & B/D" (sadomasochism and bondage and domination). By 18, Rader said his fantasies escalated into window-peeping and stealing women's underwear.

It wasn't until 2005 when police finally captured Rader that they discovered how deadly his fantasies became. While searching his property, authorities found a treasure trove of "trophies" from Rader's 10 victims, copies of the letters he sent police and hundreds of photographs. Some were of the people he murdered, and others were of himself in various forms of bondage. According to CNN, Rader admitted to setting up his tripod and camera in hotel rooms and taking photographs of himself, and he would often experiment with "auto-erotic activity," which involved Rader limiting his own oxygen supply to experience "a heightened feeling of euphoria' during sexual release."

These selfies — featured in Oxygen's "Snapped: Notorious BTK Killer" — gave investigators crucial insight into how family man and church leader Dennis Rader transformed into the BTK Killer. Scroll down to see what authorities discovered at his Wichita, Kansas, home and office.

[Photo: Oxygen]

Tied to a chair. Rader would often dress in women's clothing for the selfies. 

[Photo: Oxygen]

Bound in the woods. He would also conceal his face with various masks.

[Photo: Oxygen]

Hanging from a tree. In various selfies, Rader hung himself from a tree with elaborate bondage.

[Photo: Oxygen]

Hanging in women's clothes. Again, Rader dressed himself in women's clothes and bound himself.

[Photo: Oxygen]

Shed of secrets. In a shed on Rader's property, police also discovered "soiled lingerie" and sexually explicit books and magazines wrapped in plastic.

[Photo: Oxygen]

Masked in a ditch. According to CBS News, Rader would often relive "the ecstasy of the murder" by taking photographs of himself in the victims' clothing.

[Photo: Oxygen]

Covered in plastic. By recreating his crime scenes, Rader would be able to "live in that moment for years," said Capt. Sam Houston of the Sedgwick County Sheriff's Office

To hear more about the BTK Killer, watch "Snapped: Notorious BTK Killer" on Oxygen.

[Photo: Oxygen]