It also requires a willingness to lock up dangerous people, with two very clear cut cases of the Colorado movie theater and Virginia Tech killers.  The former so alarmed his university psychiatrist that she alerted the school authorities (as I recall, she was also on the responsible committee), but they dropped the issue when he withdrew a few weeks before the shooting.  In the latter, a state judge order the shooter to get outpatient treatment, but the state dropped the ball in seeing that that happened <i>or</i> reporting it to the Feds to put him on the National Instant Criminal Background Check System (NICS), which would have prevented him from buying his guns.<p>The flip side is that while mental illness is not surprisingly strongly correlated with mass shooters, the reverse is not true, the vast majority of the mentally ill are not a danger to anyone but themselves.