How does Dr. Kreizler help in the investigation?

Even though Dr. Kreizler is not supported in his work or his community, his beliefs in psychology prove to be a big help in tracking down a killer. Kreizler believes that if his team can look at the serial killer's acts, at his words, and at his chosen crime scenes, they can figure out what kind of person he truly is. By doing this, the team builds a picture of the criminal, not only of his present person, but of the person he was in the past as well.

Dr. Kreizler brings to the search for a serial killer a psychological aspect that has up to this point not been known. Investigators rarely used what we now know as forensics and especially not a profile of the killer's psychology. Investigators would view the facts as they saw them and use them to the best of their ability to find a killer. In the case of murdered and mutilated boy-prostitutes, the investigators saw no reason to investigate, deciding that perhaps it was better for society if these young men simply disappeared. Therefore, the case would not have been solved if not for Dr. Kreizler's unique investigation.

Dr. Kreizler searches for the killer, not using the few clues left at the crime scenes, but using the entire crime scene as well as other clues. Dr. Kreizler builds a picture of the killer based on his choice of water based crime scenes, his choice of victims, and his words in a taunting letter to a victim's mother. Dr. Kreizler uses psychology to hunt a killer and is successful in his endeavors, leading to what is now common practice in law enforcement agencies worldwide. This makes the psychology of murder a theme of the novel.