Even tough gun laws -- where they exist, like California -- have major gaps in effectiveness, and no state or federal system exists to keep weapons out of the hands of those most likely to turn violent, say gun control and mental health advocates.
Last week's massacre at a Connecticut elementary school could not have been prevented under current laws that prevent mentally ill people deemed a risk to themselves or others from buying weapons. The killer, a 20-year-old man with no documented history of mental illness, used guns that his mother had obtained legally.
The attack has reignited debate about how best to keep guns out of the hands of unstable, violent people. Victims' rights advocates say the current regulations need to be more restrictive and better enforced, so that people like Adam Lanza, the shooter in Connecticut, can't easily, and even legally, obtain guns.
Perhaps the biggest challenge will be determining who is violent or has a propensity toward violence. Mental health experts note that violence and mental illness are not one and the same -- most people who commit violent acts aren't mentally ill, and the vast majority of people with a mental illness are no more violent than anyone else.
Tighter gun control restrictions, no matter how well intentioned, might do nothing more than stigmatize people with conditions like schizophrenia or bipolar disorder who already are unfairly stereotyped as violent or frightening, and still miss potential killers who have no record of mental illness, mental health experts say.
"The question shouldn't be, 'Do you have a mental illness and have you been treated?' But, 'Do you have a history of violence, a history of anger?' " said Dr. John Greene, a forensic psychiatrist who teaches at Stanford University.
Still, Greene and other mental health experts said it's reasonable to ban people with a mental illness who are clearly unstable or at risk of harming themselves or others from buying weapons.
Those individuals are tracked on the National Instant Criminal Background Check System, a pillar of U.S. gun control regulation that collects reports of people prohibited from buying guns. Most people in the database are convicted criminals, but about 7 percent are banned because of mental illness.
But the database has two fundamental flaws. First, it applies to only the 40 percent of guns sold through licensed dealers, who must do background checks. And second, the registry is incomplete, especially regarding records of people who have a severe mental illness. The criteria for listing have never been clear, and vary widely from state to state, mental health legal experts say. The database can't identify people who have never been diagnosed with a mental illness or who are at risk of becoming violent.
Lanza may have had Asperger's syndrome, a form of autism, but that's considered a developmental disorder, not a mental illness. It's not associated with violence. He apparently had no record of violence or hospitalizations.
Broadening regulations -- keeping guns from people with any kind of mental illness -- would impinge on the rights of people who've never been and will never be violent, said Ron Honberg, legal affairs director for the National Alliance on Mental Illness.
If anything, mental health experts say, the spate of recent shootings underscores the lack of treatment options and the climate of stigmatization that prevents many people from getting the care they need.