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Mass murderers’ intentions difficult to predict

In a YouTube video posted the day of the shootings, Elliot Rodger said he was going to take revenge on humanity.

YouTube/Associated Press

In a YouTube video posted the day of the shootings, Elliot Rodger said he was going to take revenge on humanity.

GOLETA, Calif. — Colorado movie theater shooter James Holmes. Sandy Hook school attacker Adam Lanza. And now Elliot Rodger. All were young loners with no criminal history who went on shooting sprees, leaving distraught families in their wake.

Mass murderers tend to have a history of pent-up frustration and failures, are socially isolated and vengeful, and blame others for their unhappiness, criminologists say.

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‘‘They all display deluded thinking and a lot of rage about feeling so marginalized,’’ said James Garbarino, a professor of psychology at Loyola University Chicago.

Since mass killings are extremely rare, scholars say it is difficult to predict who has deadly intentions, let alone who will reach a breaking point and take action.

Past violence is a clue, but in Rodger’s case, police did not see him as a threat to himself or others during a welfare check weeks before Friday night’s rampage near the University of California Santa Barbara that left six victims dead and 13 injured.

Although Rodger struggled with mental health issues for years, he was able to buy guns and ammunition in a state with some of the strictest gun-control laws in the country.

Rodger’s mother and counselor became alarmed about bizarre videos he posted and alerted authorities in April. But Rodger was able to convince deputies that he was not a risk to himself or others — conditions that would have allowed them to take him into custody under California law.

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It is uncertain whether the police would have had the authority under California law to search Rodger’s home for weapons when they came to check on him.

Rodger died of an apparent self-inflicted gunshot wound to the head Friday after a shootout with deputies, ending a night of terror in the campus community of Isla Vista as the semester drew to a close.

The night apparently began when Rodger stabbed his roommates to death in their apartment. The sheriff’s office has identified the dead as Cheng Yuan Hong, 20, and George Chen, 19 — both from San Jose — and Weihan Wang, 20, of Fremont. Hong and Chen were listed on the lease as roommates. Investigators were trying to determine whether Wang was a roommate or was visiting the apartment.

The subsequent shooting rampage lasted about 10 minutes, with Rodger cruising around in his BMW firing at sorority women and strangers. Deputies found three semiautomatic handguns along with 400 unspent rounds in Rodger’s car. All were purchased legally.

The university plans a memorial Tuesday to mourn and remember the six students who were killed.

Before the attacks, Rodger left a trail of YouTube videos and a 140-page manifesto ranting against women and couples and lamenting his lack of a sex life. In his postings, Rodger, a 22-year-old community college student and son of a Hollywood director, said he was a lonely and frustrated virgin.

‘‘I’m sexually attracted to girls. But girls are not sexually attracted to me. And there’s a major problem with that — a major problem. That’s a problem that I intend to rectify. I in all my magnificence and power, I will not let this fly. It’s an injustice that needs to be dealt with,’’ Rodger said in a video.

The attack played out largely as Rodgers sketched it in public postings. He said he would start by ‘‘silently killing as many people as I can around Isla Vista by luring them into my apartment through some form of trickery.’’ He said he would knock them out with a hammer, and slit their throats.

Pinpointing a mass killer ‘‘is not an exact science. We don’t have a foolproof way of predicting’’ who will turn violent, said Risdon Slate, a professor of criminology at Florida Southern College.

Those who study mass murderers say most lonely and angry people do not commit violence.

‘‘We can point to all the warning signs we missed. But they’re yellow flags. They’re not red flags until blood is spilled,’’ said James Alan Fox, a criminal justice professor at Northeastern University.

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