(Scott Utterback | Courier Journal | AP) In this undated file photo, 17 crosses bearing the names and ages of those killed in last month's shooting at Marjory Stoneman Douglas High School in Parkland, Fla., were hung overnight from a Louisville billboard that advertises a local gun show. The vast majority of mass shooters have acquired their firearms legally with nothing in their background that would have prohibited them from possessing a gun. But there have been examples of lapses in the background check system that allowed guns to end up in the wrong hands. Very few states also have a mechanism to seize firearms from someone who is not legally allowed to possess one.