Seung-Hui Cho, who committed the Virginia Tech massacre in 2007, had been diagnosed with severe anxiety disorder as a child and placed under treatment.

But Virginia Tech was prohibited from being told about Cho’s mental health problems because of federal privacy laws.

At college, Cho engaged in behavior even more bizarre than the average college student. He stalked three women and, at one point, went totally silent, refusing to speak even to his roommates. He was involuntarily committed to a mental institution for one night and then unaccountably unleashed on the public, whereupon he proceeded to engage in the deadliest mass shooting by an individual in U.S. history.

The 2011 Tucson, Ariz., shopping mall shooter, Jared Loughner, was so obviously disturbed that if he’d stayed in Pima Community College long enough to make the yearbook, he would have been named “Most Likely to Commit Mass Murder.”

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After Loughner got a tattoo, the artist, Carl Grace, remarked: “That’s a weird dude. That’s a Columbine candidate.”

One of Loughner’s teachers, Ben McGahee, filed numerous complaints against him, hoping to have him removed from class. “When I turned my back to write on the board,” McGahee said, “I would always turn back quickly — to see if he had a gun.”

On her first day at school, student Lynda Sorensen emailed her friends about Loughner: “We do have one student in the class who was disruptive today, I’m not certain yet if he was on drugs (as one person surmised) or disturbed. He scares me a bit. The teacher tried to throw him out and he refused to go, so I talked to the teacher afterward. Hopefully he will be out of class very soon, and not come back with an automatic weapon.”

The last of several emails Sorensen sent about Loughner said: “We have a mentally unstable person in the class that scares the living cr** out of me. He is one of those whose picture you see on the news, after he has come into class with an automatic weapon. Everyone interviewed would say, Yeah, he was in my math class and he was really weird.”

That was the summer before Loughner killed six people at the Tucson shopping mall, including a federal judge and a 9 year-old girl, and critically wounded Rep. Gabrielle Giffords, among others.

Loughner also had run-ins with the law, including one charge for possessing drug paraphernalia — a lethal combination with mental illness. He was eventually asked to leave college on mental health grounds, released on the public without warning.

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Perhaps if Carl Grace, Ben McGahee or Lynda Sorensen worked in the mental health field, six people wouldn’t have had to die that January morning in Tucson. But committing Loughner to a mental institution in Arizona would have required a court order stating that he was a danger to himself and others.

Innumerable studies have found a correlation between severe mental illness and violent behavior. Thirty-one to 61 percent of all homicides committed by disturbed individuals occur during their first psychotic episode — which is why mass murderers often have no criminal record. There’s no time to wait with the mentally ill.

James Holmes, the accused Aurora, Colo., shooter, was under psychiatric care at the University of Colorado long before he shot up a movie theater. According to news reports and court filings, Holmes told his psychiatrist, Dr. Lynne Fenton, that he fantasized about killing “a lot of people,” but she refused law enforcement’s offer to place Holmes under confinement for 72 hours.

However, Fenton did drop Holmes as a patient after he made threats against another school psychiatrist. And after Holmes made threats against a professor, he was asked to leave campus. But he wasn’t committed. People who knew he was deeply troubled just pushed him onto society to cause havoc elsewhere.

Little is known so far about Adam Lanza, the alleged Newtown, Conn., elementary school shooter, but anyone who could shoot a terrified child and say to himself, “That was fun — I think I’ll do it 20 more times!” is not all there.

It has been reported that Lanza’s mother, his first victim, was trying to have him involuntarily committed to a mental institution, triggering his rage. If true — and the media seem remarkably uninterested in finding out if it is true — Mrs. Lanza would have had to undergo a long and grueling process, unlikely to succeed.

As The New York Times’ Joe Nocera recently wrote: “Connecticut’s laws are so restrictive in terms of the proof required to get someone committed that Adam Lanza’s mother would probably not have been able to get him help even if she had tried.”

Taking guns away from single women who live alone and other law-abiding citizens without mental illnesses will do nothing about the Chos, Loughners, Holmeses or Lanzas. Such people have to be separated from civil society, for the public’s sake as well as their own. But this is nearly impossible because the ACLU has decided that being psychotic is a civil right.

Consequently, whenever a psychopath with a million gigantic warning signs commits a shocking murder, the knee-jerk reaction is to place yet more controls on guns. By now, guns are the most heavily regulated product in America.

It hasn’t worked.

Even if it could work — and it can’t — there are still subway tracks, machetes, fists and bombs. The most deadly massacre at a school in U.S. history was at an elementary school in Michigan in 1927. It was committed with a bomb. By a mentally disturbed man.

How about trying something new for once?