Seven Taliban gunmen stormed the Army public school with bombs strapped to their bodies Tuesday, officials said.
About 100 children were in an auditorium when the militants stormed in. Following the attack, chairs were left upturned, with blood staining the floor.
Torn notebooks, pieces of clothing and children's shoes were scattered about.
A principal’s office was torn to pieces by one of the militants, the furniture ripped apart.
Bullets flew across classrooms, with the Taliban going room to room, hunting students. The building’s walls are pock-marked with bullet holes.
“They started shooting them indiscriminately,” Pakistan military spokesman Maj. Gen. Asim Bajwa said. “And after that they started spreading into other wings of the school where they were contained.
"This is not a human act," he added. "This is a national tragedy."
The Taliban attackers died, but it was unclear how. As news of the attack spread, terrified parents rushed to the school, waiting to hear if their sons or daughters survived.
“This act of terror angers and shakes all people of conscience, and we condemn it in the strongest terms possible,” Kerry said.
Pakistan’s government declared a three-day mourning period starting today, with funerals beginning for some of the children killed in the massacre.
The Associated Press contributed to this report.