Anne Marie Hochhalter, who was paralyzed during the 1999 attack on Columbine High School, is pictured in this undated file photo close to her high school graduation. (Photo by Helen H. Richardson ...

Anne Marie Hochhalter, who was paralyzed during the 1999 attack on Columbine High School, right, holds Sue Townsend’s hand during a vigil to honor all of those impacted by the Columbine High ...

The April 20, 1999, Columbine shooting, which killed 13, shocked the nation. Here's what has and hasn't changed in school shootings in the 20 years since.