Up until April 20, 1999, there had only been six other instances in American history in which five people or more had been killed during an attack on a school.
But in the 20 years since two students murdered 15 people at Columbine High School in Colorado, there have been nine more school shootings that resulted in the deaths of five students or school employees.
Read more about each of the the shootings in the list below.
The list below does not include the 200-plus shootings at schools since April 20, 1999 in which less than five people were killed.
