In 2010, Omar Thornton killed eight colleagues in Manchester, Conn., before killing himself. Private employers used to create their own rules about guns on their property. But over the past five years, many states have adopted laws that allow employees to keep firearms in their vehicles at work. Douglas Healey/Getty Images hide caption

itoggle caption Douglas Healey/Getty Images

The Armatix smart gun is implanted with an electronic chip that allows it to be fired only if the shooter is wearing a watch that communicates with it through a radio signal. It is not sold in the U.S. Michael Dalder/Reuters/Landov hide caption

itoggle caption Michael Dalder/Reuters/Landov

Flags fly at half-staff Tuesday after the deadly shootings at the Navy Yard in Washington, D.C. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP