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

President Barack Obama greets people outside the Old Market Deli in Cannon Falls, Minn., on the Aug. 15, during a Midwest bus tour. The same day he said he likes the term "Obamacare."

Samantha Appleton/The White House hide caption

itoggle caption Samantha Appleton/The White House