Colorado educators take part in a concealed carry course in Englewood, Colo., on Nov. 8. The course is open to all state school employees. Participants who complete the training are eligible to apply for a permit to carry a handgun. MATTHEW STAVER/Landov hide caption

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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

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More than half of American women now say owning a gun protects people from becoming victims of crime, according to Pew. Here, a woman carries a rifle at a gun rights rally at the Utah State Capitol last year. George Frey/Getty Images hide caption

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President Obama speaks in April after the Senate rejected a bill that would expand background checks on guns. The White House is seeking to clarify rules that bar firearm possession due to mental health problems. Manuel Balce Ceneta/AP hide caption

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New York City Mayor Michael Bloomberg displays a confiscated AR-15 assault rifle in East Harlem as District Attorney Cy Vance (right) looks on during a news conference in October 2012 in New York City. Mario Tama/Getty Images hide caption

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Former Rep. Gabby Giffords and her husband, Mark Kelly, at a gun show in Saratoga Springs, N.Y., in October. Giffords was shot in the head in a 2011 mass shooting in Tucson. She and Kelly have since founded a political action committee to push for tougher gun laws. Tim Roske/AP hide caption

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