When alleged gunman Adam Lanza, 20, opened fire inside the Sandy Hook Elementary School in Newtown, Connecticut the world watched in horror. With an estimated 26 people confirmed dead—including 20 children—it is the second-deadliest school shooting in U.S. history. The outbreak brings back chilling memories of similar school shootings that have rocked the nation through the years. From Columbine to Virginia Tech, here is a look back at the worst incidents in recent history.