GOLDEN, CO – FEBRUARY 26: A bag of 9mm shell casings are shown on display at the Jefferson County Fairgrounds February 26, 2004 in Golden, Colorado. Columbine students Eric Harris and Dylan Klebold killed 13 people at Columbine High School April 20, 1999 in Littleton, Colorado in the worst school shooting in U.S. history. (Photo by Mark Leffingwell/Getty Images)