Colorado Department of Corrections chief Tom Clements was shot and killed when he answered the front door at his home in Monument, Colo. Colorado Department of Corrections via AP hide caption

itoggle caption Colorado Department of Corrections via AP

Chris Colwell, director of emergency medicine at Denver Health, has treated victims from two of the deadliest mass shootings in the U.S. He says he's deeply disturbed by how easy it is to get guns. Barry Gutierrez/for NPR hide caption

itoggle caption Barry Gutierrez/for NPR

People at a Denver party early this month celebrate the recognition of an amendment to the Colorado constitution legalizing recreational use of marijuana. Brennan Linsley/AP hide caption

itoggle caption Brennan Linsley/AP

Police gathered at the east entrance of Columbine High School in Littleton, Colo., on April 20, 1999, after two teen students went on a shooting rampage before turning the weapons on themselves. Mark Leffingwell/AFP/Getty Images hide caption

itoggle caption Mark Leffingwell/AFP/Getty Images

A Mitt Romney supporter holds up a sign showing her love for Romney and Denver Broncos quarterback Tim Tebow at a rally for the GOP presidential candidate at Arapahoe High School this week in Centennial, Colo. Marc Piscotty/Getty Images hide caption

itoggle caption Marc Piscotty/Getty Images