The killer leaves behind a few clues; among them are a mysterious substance which fluoresces under laser light, which was later on proved to be from a liquid soap which the killer used to wash his hands, traces of semen, and in the vicinity of the last murder, an unusual smell. Scarpetta and Marino work with FBI profiler Benton Wesley to attempt to piece together a profile of the killer. Initial evidence appears to point to the fourth victim's husband, but Scarpetta suspects otherwise despite Marino's insistence. The book references DNA profiling as a relatively new technique, and characters briefly bemoan the lack of a criminal DNA database which could provide better leads to suspects, given available evidence.