21 years since shooting at Columbine High School in Colorado

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TUCSON – Today marked the 21st anniversary of the mass shooting at Columbine High School in Littleton, Colo.On April 20, 1999, two Columbine High School seniors went on a shooting spree inside the school, killing 12 of their classmates and one teacher before committing suicide.At least twenty others were wounded.At the time, the Columbine shooting was the worst high school shooting in U.S. history.Since then, mass shootings have occurred with shocking