From Ted Bundy to the BTK Killer, many of the nation’s most notorious serial killers got their start peering into windows as a teen before escalating to far deadlier crimes.

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Serial killers are known for heinous acts of violence that strike fear across the country — but many of the nation’s most well-known serial killers got their start with a far more innocuous crime.

Investigators say Ted Bundy, suspected Golden State Killer Joseph DeAngelo, and Dennis Rader, known as “BTK,” all honed their criminal skills as Peeping Toms during their teen and early adult years. Other killers such as Derrick Todd Lee and Philip Hughes were also known for peeping on unsuspecting victims.

“It’s a violation of privacy and it does lend itself to power and control,” Scott Bonn, PhD, told Oxygen.com. “Bundy and BTK were all about power and domination and control.”

The Beginning Of A Natural Evolution

Bonn, who is a criminologist and author of the book “Why We Love Serial Killers: The Curious Appeal of the World’s Most Savage Murderers,” said voyeurism or peeping on others can be part of a natural evolution of criminal activity.

“You don’t just wake up one day and suddenly say I am a serial killer. It doesn’t work that way,” he said.

As Paul Holes, a retired investigator for the Contra Costa County District Attorney’s Office, describes it, to evolve into a serial predator, a person must overcome a series of increasing “barriers to offend.”

Holes, who was instrumental in catching the suspected Golden State Killer, said the average person is often uncomfortable with the idea of walking onto a neighbor’s yard without permission or peering into someone’s windows, which creates a societal “barrier” from doing those activities.

Burgeoning serial killers have to overcome those same feelings and often begin with acts that have less significant societal barriers than other illegal deeds. So, someone may begin as a Peeping Tom, watching someone from outside their home without their knowledge before escalating to more serious crimes such as going into the house while no one is home and stealing underwear or another item they find sexually arousing.

Serious offenders, Holes said, are those who escalate again and break into a home when someone is there.

“That’s obviously a huge barrier that they’ve crossed,” he said.

That’s the reason burglary is considered such a serious crime and is reported to the FBI’s Uniform Crime Reports — a program that tracks some of the most significant criminal offenses — because it shows “somebody is willing to take a huge step in criminality,” Holes explained.

An example of this criminal escalation can allegedly be seen in Joseph DeAngelo, authorities say.

Investigators believe DeAngelo — who was arrested in 2018 for 13 murders linked to the Golden State Killer — first began his alleged criminal career secretly watching others.

Authorities also suspect DeAngelo of burglarizing approximately 100 homes in Visalia between 1974 and 1975. Authorities gave the skilled prowler the name the Visalia Ransacker.

On one occasion, Holes said the suspect was seen peering into the window of a young woman’s house by her college boyfriend, who chased the suspect down before he was able to escape by making a motion like he had a weapon.

“I can guarantee DeAngelo is out peeping as a young man and many offenders do that, you know,” Holes said. “If they continue to evolve, they are the ones who are ultimately going to want to get into somebody’s house and ultimately sexually assault or kill.”

Investigators believe DeAngelo, who has yet to enter a plea in the charges against him, was likely a Peeping Tom in his teenage years — but they haven’t found anything definitive to prove that’s when it began.

Derrick Todd Lee — a charismatic Louisiana resident suspected of killing at least seven women during his reign of terror — began his foray into crime as a Peeping Tom.

Former neighbor Eddie Berry told The Times-Picayune in 2003 that Lee was caught peering into women’s windows as an early teen.

“No one called police, though,” he said. “I guess people thought he was being curious and would grow out of it eventually.”

Lee would later be arrested on Peeping Tom charges — including an occasion in 1999 when he waited outside in a woman’s bushes before following her inside to her apartment uninvited.

The Appeal Of Peeping On Unsuspecting Victims

Spying on unsuspecting victims has other advantages for violent predators as well.

Although most serial killers don’t begin killing until they reach their 20s, Bonn said their fantasies of domination, control and power typically begin in adolescence and early adulthood.

Peeping Toms are able to gain a sense of power and control by putting victims “in their sights” and watching them without their knowledge.

“It makes them feel empowered and strong and they are still not ready to kill, but it’s part of this maturation process of a fledgling serial killer,” he said.

Bonn, who has corresponded extensively with Rader after his arrest, said the admitted killer began watching women long before he ever committed 10 heinous murders in Wichita, Kansas.

“When he was stationed in the Army over in Germany, he would break and enter and break into women’s apartments and steal their underwear,” he said.

For many, there’s a sexual component to voyeuristic acts.

Convicted serial killer Phillip Hughes began his foray into crime as a high schooler prowling his Contra Costa County neighborhood streets naked at night.

“He is going out in the middle of the night nude — sneaking out of his parent’s house in the middle of the night — and going around and looking into houses and actually breaking into houses and stealing women’s undergarments,” Holes said.

Hughes was later convicted of killing three women in the 1970s and remains a person of interest in six other slayings, according to a 2011 article in The Mercury News.

Investigators believe DeAngelo also may have prowled the streets naked during the series of crimes often attributed to the East Area Rapist. According to Holes, victims in the first and third attacks reported that their attacker was naked form the waist down.

“The victim fought him in attack number three and now he’s having to run around outside in the nude and law enforcement is responding,” Holes said, adding that he later decided to scrap his habit of going nude.

Predators may also masturbate while peering into the windows of homes, further fulfilling their sexual desires, according to Bonn.

“Part of that sexual gratification is why they do this, so it fulfills a fantasy need in and of itself, but then it also does serve the purpose of helping them prepare for their next strike, their next victim,” he said.

Deadly “Training Ground” For Killers

In addition to fulfilling a fantasy, voyeuristic acts also have practical benefits for dangerous predators.

In a 20/20 special on Bundy earlier this year, former FBI profiler Brad Garrett called peeping a “training ground” for serial killers.

“The idea that Ted Bundy was involved in peeping actually makes sense because it’s basically a training ground about how you isolate people, how you watch people, how you get into houses,” he said.

Many serial killers continue to watch or “troll” for victims long after they’ve started to kill to help them identify potential targets. This covert surveillance helps them learn more about a potential target’s habits and develop a strategy to gain entry into the house undetected.

Rader would often watch women for long periods of time before he ever struck to learn more about their comings and goings, family connections, activity in the neighborhood, and the best times to attack.

“He would watch them and observe them over time before striking,” Bonn said.

Does Every Peeping Tom Escalate?

While many of the country’s most notorious serial killers got their start as Peeping Toms, experts say not all Peeping Toms become killers.

“There are offenders that just kind of stagnate at a particular step. That’s all they need to do and that could be just peeping,” Holes said. “The person is a habitual peeper but never takes to where now he’s going into the house.”

It's difficult to gauge which offenders will go on to pursue more serious crimes, but Holes said investigators typically play close attention to burglaries where there is a fetish component.

“We treat that like that person is in all likelihood going to escalate,” he said.

Other worrisome clues are offenders who have broken into homes while the homeowners are there or those who have been caught standing at a women’s bedside, even if they never touch her.

“That is a serious offender that is likely going to escalate,” Holes said.

For certain offenders, peering into the home of a stranger fuels a deeper need that just may drive them to begin to envision a more deadly crime.

“Certainly, a very, very, very small subset of all Peeping Toms are serial killers but I would say becoming a Peeping Tom or being a Peeping Tom is a very logical and natural stage in the progression of a fledging future serial killer,” Bonn said.