School gun violence and the terror it creates have riveted America again since a gunman shot and killed 17 last month at Marjory Stoneman Douglas High School in Parkland, Fla. The public outrage has produced a new wave of protests, including walkouts in schools across the country on Wednesday.

But acts of violence aren’t the only source of school terror.

For each heinous attack successfully carried out, there have been many more scores of threats in schools across the country. And while thankfully no one dies because of a threat, fake or foiled attacks can be terrifying, too.

After the Parkland attack, we reached out to students across the country, asking them to tell us about threats their schools have received. We heard from more than 100 teenagers, some describing hiding in dark closets and classrooms, sometimes for hours, as they braced for an attack.

In the five months from August to December 2017, there were at least 671 bomb or gun threats — or both — directed at American schools, according to the Educators School Safety Network, a nonprofit that provides school safety training. In the past month, since the Parkland shooting, there have been more than 465 bomb or gun threats or both.