On April 20 1999, two teenagers walked into their high school in Colorado armed with guns, bombs and grenades and killed 12 fellow students and a teacher before taking their own lives. The tragedy had a seismic impact on America, with the murderers cited as an inspiration by subsequent killers. Now, for the first time, Sue Klebold, the mother of one of the shooters, talks to Kirsty Wark about trying to come to terms with what happened at Columbine High School and her efforts to raise awareness about children's mental health issues.