The vast majority of U.S. mass shooters have acquired their firearms legally with nothing in their background that would have prohibited them from possessing a gun.
However, there have been examples of lapses in the background check system that allowed guns to end up in the wrong hands, including weapons used attacks at churches in Charleston, South Carolina, and Sutherland Springs, Texas, in recent years.
Very few states also have a mechanism to seize firearms from someone who is not legally allowed to possess one.
In 2018, there were more than 26 million background checks conducted and fewer than 100,000 people failed. Of those, the vast majority were for a criminal conviction. Just over 6,000 were rejected for a mental health issue.