How many students have experienced gun violence at school?

The Washington Post spent months determining how many children have been exposed to gun violence during school hours since the Columbine High massacre in 1999.

Beyond the dead and wounded, children who witness the violence or cower behind locked doors to hide from it can be profoundly traumatized.

The federal government does not track school shootings, so The Post pieced together its numbers from news articles, open-source databases, law enforcement reports and calls to schools and police departments.

The children impacted grew with each round of reporting: from 135,000 students in at least 164 primary and secondary schools to more than 187,000 on 193 campuses.