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.

Evidence suggests their intent was to carry out acts of violence at a school in the Spring Branch Independent School District, law enforcement said.