PHOTOS: At least a dozen are killed in Colorado theater shooting
A look at some of the deadliest mass shootings throughout history.
Dec. 14, 2012: Newtown, Conn., school shootingPeople leave the scene of a school shooting at Sandy Hook Elementary School in Newtown, Conn. Adam Lanza, 20, entered the school and shot and killed 26 people, including 20 children, and then himself. Lanza also killed his mother at their home. The guns were legally purchased by Lanza's mother.Don Emmert/AFP/GETTY IMAGES