Binding, torturing and killing victims is how serial killer Dennis Rader methodically murdered 10 people near Wichita, Kan., during a 20-year killing spree that garnered him the nickname "BTK strangler."
Like Rader, who began his work in the 1970s, several infamous serial killers have taken ritualistic approaches, either by leaving signature characteristics or repeating their steps in the killings.
Albert Desalvo, the 1960s "Boston Strangler," was convicted of strangling 13 women and then tying nylon stockings in a bow around their necks. Jeffrey Dahmer, who killed 17 men between 1978 and 1991, routinely took body parts from his victims and stored them in his refrigerator. The Zodiac Killer, who got his name from the symbols he drew on notes to the media, is still unidentified, despite having killed at least five people between 1968 and 1969 in California.
So, when two New York City detectives found smiley faces painted at the crime scenes of more than 40 young men — typically high achieving college students initially presumed to have drowned in 25 cities in 11 different states — they began to wonder if they had uncovered yet another overtly proud serial killer.
"We believe they were specifically leaving a clue for us or anyone who was paying attention to these drownings, that the cases were ultimately linked," Detective Kevin Gannon told ABC's "Good Morning America."
The detectives found the smiley faces in Ohio, Pennsylvania, Indiana, Wisconsin and Iowa.
While there has yet to be a sexually-driven motive identified in the smiley face case, criminologists told ABCNEWS.com that it's an example of the kind of signature typically left by psychopathic killers who derive sexual arousal from their killings and are so proud of their murders that they'll do anything they can to get credit for them.
"Signatures are ritualistic crime scene behaviors that are done for psychosexual gratification," said Louis Schlessinger, a professor of forensic psychology at John Jay College of Criminal Justice in New York. "It goes beyond what is necessary to kill that person because killing alone is not enough."
"The most typical types or signature is leaving the woman in some sort of sexual degrading position," said Schlessinger, who added that he has studied cases where killers put dirt in the mouths of their victims or sodomize them with foreign objects.
These crime scene signatures, according to Schlessinger, are often indications that the killer wants to brag about his accomplishment.
"They're trying to show the police that they're very proud of what they've done and that they feel tremendous," Schlessinger said. "It's arousing to them to feel control and power."
Jack Levin, author of "Serial Killers and Sadistic Murderers," agreed.
"The signature that they leave is often designed to give them what is otherwise missing in their drab and dreary lives — they feel important and special. They've achieved national prominence, even anonymously, and it makes them feel as though they have succeeded."
Levin told ABCNEWS.com that while many serial killers don't leave signatures, many decide to do so, not only to achieve fame, but also to tease law enforcement.
"To a serial killer, torturing and killing victims are great accomplishments and is something that he wants the world to know," Levin said. "At the same time, he leaves the signature at the crime scene to taunt the police.