Every one of the country’s mass shooters since January 2009 could have slipped through NICS, according to a July 2014 study by the gun control organization Everytown for Gun Safety.

Millions of people who have been forced into hospitals for mental health issues are left out of the system. Only about 30 percent of the 4.4 million estimated mental health records in the U.S. over the last two decades can be found in NICS, according to research compiled in 2012 by the National Consortium for Justice Information and Statistics and the National Center for State Courts.

Those whose names are found in the system are unlikely to try to buy a gun. Out of all gun purchases blocked by the FBI over the last 16 years, fewer than 2 percent were because of mental health status. That amounts to 14,613 blocked sales since 1998.

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