Nobody could have fathomed it; it didn’t seem real. Sixteen years later, the nation still tries to make sense of the senseless tragedy that occurred at Columbine High School in Littleton, Colorado. On April 20, 1999, Eric Harris and Dylan Klebold — only 18 and 17 years old, respectively — walked into school wielding an arsenal of firearms, and committed one of the worst school shootings in American history.