“These are not acts that a person is compelled to do,” said J. Reid Meloy, a forensic psychologist and professor of psychiatry at the University of California, San Diego. “They are intentional and predatory. There is choice, capacity and opportunity that is exercised.”

Any number of factors can contribute to a dormant stretch. An extensive 2008 study on serial murder for the F.B.I.’s National Center for the Analysis of Violent Crime found that killers may quiet down when they find other outlets for their emotions. The study mentioned Dennis Rader, known as the BTK Killer, who murdered 10 people from 1974 to 1991, but had no other victims before being apprehended in 2005. “During interviews conducted by law enforcement, Rader admitted to engaging in autoerotic activities as a substitute for his killings,” the report said.

Other killers might have changed behavior after moving away from the original epicenter of activity. Ted Bundy mutilated and murdered perhaps more than 30 young women in the 1970s. Yet there were stretches along his peripatetic travels when he was not associated with murders in those areas.