Nikolas Cruz appeared in a Broward County, Fla., court this week. (Mike Stocker/EPA-EFE/Shutterstock)
Arnold Arluke, an emeritus professor of sociology and anthropology at Northeastern University, is author of “Just a Dog: Understanding Animal Cruelty and Ourselves.”

Inevitably, a mass shooting is followed by a public account of the “warning signs” the killer may have evinced in the months and years before the attack: bullying, isolation, vacant gazes, loss of parents, violent ideations, school expulsion, depression, explosive outbursts. One of the most common, though — hurting animals — is also the only one that is sometimes illegal and, therefore, the only one that could theoretically be used to bring troubled youths into the mental and criminal justice systems before they do something horrible.

Before he confessed to killing 17 people at Marjory Stoneman Douglas High, Nikolas Cruz, too, allegedly abused animals. In elementary school, Cruz began shooting squirrels and chickens; as a teenager, he is said to have killed frogs, tried to maim a neighbor’s baby potbelly pigs and tried to crush animals trapped in rabbit holes. On Instagram, he boasted about killing animals and posted images of dead ones.

Since the 1960s, some criminologists, psychiatrists and other investigators who study serial killers and mass murderers have claimed that animal cruelty is a possible predictor of future violence. But many children treat animals maliciously, even kill them, and a vanishing percentage become mass killers. In a study I did with Jack Levin of Northeastern University, we found that 28 percent of 260 undergraduates admitted to having abused animals when they were children. Other social scientists report up to 35 percent of college students who recall former cruelty. On a national scale, these findings suggest that hundreds of thousands of children harm animals at some point in their youth. It seems to be a kind of dirty play that many children consider to be no worse than cursing, roughhousing and making offensive sexual remarks or racial epithets.

How useful, then, is this warning sign? It would be vital to know how often school shooters commit animal abuse and what kind of cruelties do they mete out, in terms of victims, methods and frequency? How are their abuses different from the hundreds or thousands of routine cases? Which instances of animal abuse are warnings of a possible school massacre, and which are merely false positives?

I recently set out to answer these questions with my co-author, Eric Madfis of the University of Washington. We looked at 23 school shooters from 1988 to 2012 and found reports of prior animal cruelty in the histories of 10 of 23 (43 percent) school shooters, a rate almost as high as in the backgrounds of serial killers. These two categories of people treated animals differently than ordinary folks. Ninety percent of our animal-abusing school shooters committed cruelty in an up-close and personal manner — strangling, bludgeoning, burning or mutilating — much like serial killers.

No wonder this kind of behavior often turns up in the reporting about the shooters. Columbine killers Eric Harris and Dylan Klebold, for instance, boasted about mutilating animals for fun, a type of cruelty that required hands-on control of the victim and tactile cutting. Kip Kinkel, who fired on students in Oregon’s Thurston High, allegedly decapitated and immolated cats, dissected live squirrels, blew up cows and put firecrackers inside gophers and cats. Other school shooters burned, drowned, kicked or smashed their animal victims. Only a minority of school shooters (10 percent) apparently employed more remote methods to harm or kill animals, such as shooting them, which did not require them to be near, let alone touch, their victims. Seven out of 10 school shooters and serial killers who hurt animals targeted those — mostly dogs and cats — from outside their homes and neighborhoods. (One notable exception was Luke Woodham, a Mississippi school shooter, who described in his journal how he tortured and killed his own dog by beating it in a plastic bag and setting it on fire.)

Would this more specific pattern of abuse have identified Parkland’s Cruz as a high risk for a massacre? Probably. Although his animal abuse didn’t exactly match the characteristics we found were most likely to predict school shooters, it was flagrant and involved multiple episodes. Moreover, although Cruz did not torture dogs or cats, he allegedly tried to injure or kill potbelly pigs, which are often treated as family pets and used up-close-and-personal methods. He also targeted a few victims with unknown owners or no owners.

Future tipsters and law enforcement workers should be cautious, though, about assuming that animal abuse is a necessary precondition for a shooter. In fact, a systematic investigation of targeted school shootings revealed that there simply “is no accurate or useful ‘profile’ of students who engaged in targeted school violence,” whether demographic, psychological or social. In many cases, animal abuse isn’t, and when it’s there, it takes a specific form. And even with research-based checklist items such as animal abuse, some cases will inevitably not be red-flagged. If we flag every incident of animal abuse, our mental health and justice systems would be overwhelmed with tens of thousands of cases to review.

It is understandable that people will call for more reliable checklist items that can spot future killers and prevent school massacres. As Adam Lankford, a criminologist at the University of Alabama, lamented: “In many recent cases, law enforcement has known about a future mass shooter before his attack but failed to recognize the threat he posed. With more accurate warnings signs, they may be able to prevent the next tragedy.” Given the complexity of predicting any human behavior, let alone extreme killing, more precise warning signs will not identify every future shooter. But our research offers hope for spotting warning signs of — and thus preventing — at least some school shootings.