Jack Levin, author of the 2008 book, “Serial Killers and Sadistic Murders — Up Close and Personal,” said he found that more than half of serial killers he studied had abused animals, many of them maximizing the suffering of dogs and cats.

He did a study comparing 150 animal abusers to those who never abused animals, he said. “The animal abusers were five times more likely to commit human violence, such as rape and robbery. They were also four times more like to commit property offenses.”

The common belief is people “graduate” from animals to people, he said, “but we also found they may start with people and end with animals — or do both at the same time.”