Police say the suspect had a ghost gun, possibly made with a 3D printer, when he was arrested on Monday. Here's what to know about the increasingly widespread weapons and efforts to regulate them.

Ghost guns really popped into the public consciousness in 2013 when John Zawahri opened fire on the campus of Santa Monica College in California, killing six people, including his father and brother. Zawahri, who was later shot and killed by police, had assembled an AR-15-style weapon after failing a background check at a gun dealer.