People with histories of mental illness and a proclivity toward violence are not supposed to be able to purchase firearms in the United States. But in practice, say experts, a patchwork of state and federal laws only apply to people who have been institutionalized or deemed by authorities to be dangerous.

Federal law mandates background checks meant to keep convicted felons and people with mental illnesses from legally acquiring guns, but the rules only apply to people who buy weapons from licensed dealers -- meaning that people, including those with mental illnesses, can buy what they please from other individuals at gun shows or elsewhere. At the state level, standards vary and are bedeviled by poor tracking and a lack of coordination with federal authorities, making enforcement spotty.

In short, no effective system exists that can prevent those with mental illness from getting hold of deadly firearms or reliably predict who may act out in violence.

The underlying goal of laws restricting who can buy guns is to keep guns out of the hands of the people most likely to commit violent acts. By that measure, the laws aren't working, said Paul Appelbaum, the director of the Division of Law, Ethics and Psychiatry at Columbia University's department of psychiatry.

"What we've been pursuing here has been not just a failed strategy, but a strategy that is unlikely ever to be implemented successfully," Appelbaum said. "We're both sweeping up lots of people about whom we probably shouldn't be worried and omitting many people about whom we should."

Broadening the mental health criteria for exclusion from gun ownership wouldn't address massacres or the gun violence that occurs daily in the United States, Appelbaum said, but better screening and reporting of people with mental illnesses already restricted by the law could help. However, Appelbaum added, that additional reporting wouldn't address what he believes to be the underlying problem: how easy it is to get guns in the U.S.

"We've been trying to identify dangerous people in the hope that that could avoid our having to do something about the wide availability of dangerous weapons," Appelbaum said.

These laws are limited in reach to people who have been institutionalized or subject to legal sanctions related to their mental health conditions. People with longstanding mental health problems or histories of violence who haven't been processed by the legal system wouldn't fall under these rules.

Federal laws dating to 1968 forbid anyone from owning a firearm who has been designated by a court or some other official body as a danger to themselves or others, who has been involuntarily committed to a mental health facility or who has pleaded insanity in a criminal trial. Like all federal gun laws, these restrictions only apply to firearms purchased by licensed dealers and subject to background checks, not private sales. Poor record-keeping and a lack of coordination between state and federal governments also result in gaps in the database used for background checks.

At the state level, gun control laws range from simply conforming to the federal standard to more restrictive rules, such as a California law that requires psychotherapists to report threats of violence to the state and enforces a six-month ban on patients' gun ownership. Some state laws apply only to handguns, while others enforce restrictions only against people seeking permits to carry concealed firearms.

But even with better enforcement, these federal and state laws may not have stopped the murders of 20 children and six adults in Newtown, Conn., on Friday. Adam Lanza, 20, may have been diagnosed with Asperger's syndrome, a condition not generally associated with violent acts, and wasn't subject to these legal restrictions. Moreover, the firearms used in the massacre were legally purchased and owned by his mother and one of his victims, Nancy Lanza, 52.

James Holmes, the 25-year-old who allegedly murdered 12 people in an Aurora, Colo., movie theater this year, also purchased his guns legally because his mental health treatments didn't merit a prohibition against firearms ownership under the law. Jared Loughner, 24, who murdered six people and shot then-Rep. Gabrielle Giffords (D-Ariz.) in Tucson last year, passed background checks for the same reason.

The most significant expansion of federal regulation over gun ownership in decades was the so-called Brady Bill of 1993, which established a national database of background information about people who may be subject to limitations on their rights to possess firearms. Gun dealers are required to submit prospective customers to the database to be screened for criminal records, qualified diagnoses of mental illnesses and other factors that would prevent them from buying firearms.

As of Dec. 31, 2011, the National Instant Criminal Background Check Systems Index included 7.3 million active records of people forbidden to own guns by federal law, 1.4 million of which relate to people determined under the law to have mental illnesses that disqualify them from possessing firearms, according to a report issued by the FBI.

After 32 people were killed at Virginia Tech in 2007, Congress put pressure on states to submit records about mental health to the federal database or risk losing federal crime-fighting dollars. More than twice as many mental health records are in the databases then there were in 2008, according to the Brady Campaign to Prevent Gun Violence.

In contrast to Adam Lanza and Holmes, 23-year-old Virginia Tech shooter Seung-Hui Cho should have been stopped from buying guns under the laws in place at the time. Virginia, however, failed to submit his mental health records to the federal background-check database. Like Lanza, Cho committed suicide at the scene of his crime.

Despite improvements to the background-checks database, gaps remain, according to a November 2011 report by Mayors Against Illegal Guns, a coalition of more than 700 city officials whose co-chairman is New York Mayor Michael Bloomberg, an outspoken supporter of stricter gun control laws.

Millions of records on people with mental illnesses or substance abuse problems that should prevent them from buying guns are missing from the federal database because states aren't sharing enough information, the report concludes. Twenty-three states and the District of Columbia forwarded fewer than 100 mental health records to the background check system. Alaska, Delaware, Idaho and Rhode Island hadn't submitted any as of Oct. 31, 2011.

Background checks themselves are even controversial among supporters of gun rights. A National Rifle Association-backed bill to end criminal background checks for many people trying to buy guns passed the Michigan House of Representatives last year, for example, though the state Senate rejected the bill last week. The NRA plans to push for the change in law again next year, the organization said last Wednesday -- two days before the Newtown shootings.

People diagnosed with mental illnesses who aren't violent may be caught up in the prohibitions. Accurately predicting any kind of human behavior is imperfect and makes a shaky foundation for public policy, Appelbaum said. The leading indicators of a potential for violence -- among those with and without diagnosed mental illnesses -- include being young, male and having a substance abuse problem, Appelbaum said. Applying those restrictions would prohibit a large portion of society unlikely to commit violent acts from owning guns, he said.

While sometimes failing to keep violent people from obtaining the firearms, these laws also unfairly tar people with mental illnesses as dangerous, said Ronald Honberg, national director for policy and legal affairs at the National Alliance on Mental Illness. Fear of putting their names into an FBI database might have a "chilling effect" that would discourage individuals from seeking mental health services and would perpetuate the stigma that society needs to be protected from people with mental illnesses, he said.

"It is legitimate to limit the right of gun ownership to people who are violent or potentially violent" whether they have a mental illness or not, he said. "Most people with mental illnesses are not violent."

CORRECTION: An earlier version of this story indicated that all firearms purchases at gun shows are not subject to background checks. Licensed dealers must run background checks. Private sellers at gun shows and elsewhere do not need to do so.

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