As they went along, the profilers kept refining their methods. Sometimes they had to be creative to get the information they sought. They soon learned about the problems with self-report interviews, when some of the men bragged about brutal deeds, or those who were psychopathic played games and even lied. Nevertheless, these were the men who were there when the crime was committed—the only living witness, in most cases--and they were the ones who could tell the tale, so there were advantages to these interviews as well.

To get as much information as possible, the profilers usually did extensive research on a subject before talking with him. That way they showed some respect that the killer might enjoy, as well as knowing when his story deviated from the facts. Despite the brutality of many of the crimes, the agents realized that it was important to convey nonjudgmental interest in the subject's world.

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The initial study, meant to include 100 convicted offenders, compiled data from only 36, but it still proved helpful. Afterward, the interviews continued and other agents got involved. Ressler estimated that he had done more than 100, and he describes this with John Wayne Gacy and Jeffrey Dahmer in I Have Lived in the Monster. Gregg McCrary, who joined the unit in 1985, added to the database with interviews such as the one he describes in The Unknown Darkness with serial bomber and genius forger, Mark Hoffman.

When all was said and done, from this sample of subjects, the researchers gained statistical information that was useful for developing profiles in the late 1980s. One third of the offenders were white, nearly half had a parent missing from the home growing up (usually the father), three-fourths reported having had a cold or negligent parent, a majority had a psychiatric history as well as a history of unsteady employment, the mean IQ was bright normal, three-fourths had paraphilias, and the same percentage reported some form of physical or psychological abuse. The sample was too small to draw significant conclusions, but it was a good start.

Years later, Roy Hazelwood also developed a study, with Janet Warren of the wives and girlfriends of serial sadists, the results of which he published in the third edition of Practical Aspects of Rape Investigation. Some of them, too, were serial killers, and their stories contributed new knowledge to the database.

Even before the initial prison study was completed, something else happened to put profiling on the map.