Prosecutors have said the 17-year-old told several students about his plans to shoot up South Portland High School and asked at least one other student to help.
Extreme niche startribune.com - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from startribune.com Daily Mail and Mail on Sunday newspapers.
Maine's juvenile justice system defaults to privacy, but a South Portland case has frustrated prosecutors, who say the community has a right to know the threats.
The 17-year-old will be held at the Long Creek Youth Development Center for at least a week. Prosecutors say he participated in hateful social media group chats.
After a student was arrested in April and accused of threatening the school, a lockdown last month reignited some teachers' frustration with how little they know about his arrest.