The FBI arrested two teenage girls plotting a “mass casualty attack” using pipe bombs and guns targeting Memorial High School in Harris County, Texas, police said.

The two girls, ages 15 and 16, planned on placing pipe bombs around the school and using guns to then pick off panicking students and teachers, the Montgomery County Sheriff’s Office alleged.

Two students are now in custody after authorities identified them as being in the planning stages of a mass casualty attack against Memorial High School.