says it has stopped a potential disaster. a school shooting at a high school and elementary school. they've arrested a teenager in maryland who they say had detailed plans to target both his high school and where he went to elementary school. >> reporter: tonight, the fbi along with state and local authorities in maryland say they thwarted 18-year-old alex ye's plans to shoot up his high school and his former elementary school after authorities discovered what they called a manifesto with detailed plans to commit mass murder. >> ye writes about targeting his former elementary school because little kids make easier targets. he strategized on how to access the easiest classrooms in his high school and how he would be able to sneak a gun into the school. >> reporter: ye, whose legal name is andrea but uses male and female pronouns and goes by alex, was known to authorities as having a history of mental health issues and an obsession with school shootings.