After every shooting incident and act of public violence, we tend to circle around the same questions: "Who was he? What was her motivation?"

In trying to untangle the mystery, what we're really asking is if anyone saw this coming. We wonder whether she was bullied at school or passed over for promotions, or whether he was seeking revenge or battling mental illness.

But in the rush to get answers, we don't often glean much insight. How many times have you heard a post-tragedy TV interview with a friend or neighbor describing the perpetrator as a person who seemed normal or kind of kept to himself? So, we collectively shake our heads, sigh and try to move on. Until the next incident, when we do it all over again.

In the last few weeks in Washington, D.C. alone, a woman, reportedly suffering from post-partum depression with psychosis, led Capitol Police on a high-speed chase that ended in her being shot to death, and a man who thought he was being controlled by electromagnetic waves went on a shooting rampage at Washington Navy Yard and killed 12 people. Amid these stories, along with jury preparations in the trial of Aurora, Colo. theater shooter James Holmes and the approaching anniversary of the Newtown massacre when a 20-year-old shot 20 children at Sandy Hook Elementary School, U.S. News revisited the routine questions around these crises. And here's what we learned: We don't know the answers because we're not asking the right questions.

Most Americans aren't trained in threat detection, nor is it a simple science. You can't exactly predict a massacre. You can, however, spot the signs of someone in mental distress and alert someone to help prevent another tragedy.

For starters, if you see something, say something. Remember that slogan? It was first used in a New York City crime-fighting campaign and later by the federal government in its effort to stop terrorism - but the same mindset should apply to spotting disturbing behavior, says Marisa Randazzo, former chief psychologist for the U.S. Secret Service and managing partner with an Alexandria, Va. firm that provides guidance and training in threat assessment and management. Nearly all of the school shooters of the last decade had "seriously troubled or alarmed" one adult, and three-quarters of them alarmed three or four adults, says Randazzo, who authored a guide on campus safety in the wake of the Virginia Tech shooting.

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Recent cases have also exemplified the role of mental illness in motivating perpetrators of violence. Jared Loughner, whose 2011 Tucson shooting wounded former Arizona Congresswoman Gabrielle Giffords and killed six others, reportedly frightened classmates with his outbursts in school and had been diagnosed with schizophrenia. Miriam Carey, who took her 1-year old daughter on a drive that turned into a high-speed chase on Capitol Hill, had delusions President Barack Obama was communicating with her.

"People with serious mental illnesses have a higher rate of violent behavior than the general population and yet, at the same time, their mental illness accounts for only a small percentage of violence in the United States," says Paul Appelbaum, past president of the American Psychiatric Association and professor of psychiatry at Columbia University. He cites the classic examples of bar fights, brought on by anger and aggravated by alcohol, and notes that those at the greatest risk for violence - young men, 20-somethings and those suffering setbacks, - casts too wide a net to identify prospective criminals. While only a small percentage of mentally ill people act out in violence, "a disproportionate amount of the mass violence that occurs seems to be linked to mental illness," he says.

Appelbaum argues for a better network of mental care to help spot and support the mentally ill, and some such efforts are underway in response to the spate of shootings in this country. A program developed in Australia called Mental Health First Aid now has migrated to the United States with an official certification program to instruct people in emergency response to a mental health crisis.

But to prevent a crisis from happening in the first place, experts say the best bet is to pay attention to behavior. As to the specific behavior, that's harder to say. It's something that "makes you stop and think and worry about the person," Randazzo says. That might include an obsessive interest in violence or a school essay riddled with disturbing content or even telling someone about a dangerous scheme. Behavior can't predict a certain outcome - "the same person would do something different in different situations" - but it can provide details for a composite picture that a professional can assemble and assess, she says.

If you experience a visceral reaction to someone a la the hair standing up on the back of your neck, that's a good indicator of possible danger - but often, people misread each other, says Mary Ellen O'Toole, a retired FBI agent profiler and author of "Dangerous Instincts: How Gut Feelings Betray Us." Why? Because you need a baseline of behavior to recognize whether someone strays from his or her norm. Otherwise, it's too easy to confuse your biases with your gut, she explains. "We believe that we're born with this inner barometer that tells us if somebody is safe or nonthreatening," she says. But too often, "we look at the trappings of normalcy," like where someone went to college, "instead of the behavior that they are exhibiting right in front of us."

[Read: Could You Become a Victim of Dating Violence?]

Erratic behavior, people speaking in ways that are hard to understanding or expressing delusional thinking - all these things should raise your antenna, Appelbaum says. But most often, these patterns are detected by friends and relatives, who must grapple with whether to report a loved one to authorities. O'Toole acknowledges the pain in this decision but advises those who notice aberrant behavior to report it by contacting someone who can help such as the person's primary doctor, psychologist or psychiatrist, local psychiatric hospital or 911.

[See: U.S. News Doctor Finder.]

Spotting such signs early makes punishment much less likely, Randazzo says. As a situation worsens, your options are likely to become fewer and more severe such as arrest or an involuntary psychiatric examination.

According to Appelbaum, the attempt to identify a potentially violent criminal is nearly impossible. So, he argues for vigilance around erratic behavior and "better screening for and access to mental health services on a general basis." "Those warning signs of mental illness ought to be tended to - not because those are people who are likely to commit violence, because only a tiny number of them will," but to safeguard them from their own self- destruction.

Start by talking with the person about getting treatment and consider asking a family member or supervisor to intervene if need be, he says. To learn more about how to get help, Appelbaum suggests visiting the websites of the National Institutes of Mental Health, National Alliance on Mental Illness and the American Psychiatric Association.