Most mass shooters in the U.S. acquired the weapons they used legally because there was nothing in their backgrounds to disqualify them, according to James Alan Fox, a criminologist with Northeastern University who has studied mass shootings for decades.
But in several attacks in recent years gunmen acquired weapons as a result of mistakes, lack of follow-through or gaps in federal and state law.
Not all gun purchases are subject to a federal background check system. Even for those that are, federal law stipulates a limited number of reasons why a person would be prohibited from purchasing or possessing a firearm. Those include someone who has been convicted of a crime punishable by more than a year in prison, has a substance abuse addiction, has been involuntarily committed for a mental health issue, was dishonorably discharged from the military or convicted of domestic violence/subject of a restraining order.
In 2018, there were more than 26 million background checksconducted 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.