On a Friday morning in December 2012, a gunman burst through the doors of Sandy Hook Elementary School in Newtown, Conn., committing one of the worst mass shootings in U.S. history that left 20 children and six teachers and staff dead.
The Dec. 14 attack shocked the nation. At the White House, President Obama fought back tears; he said his visit to the school a few days later was the only time he had ever seen Secret Service members cry.
After years of increasingly violent and deadly mass shootings, Sandy Hook had a more profound effect.