Prosecutors said today they want to try John David LaDue, 17, of Waseca, Minn., as an adult for allegedly plotting an attack to kill his family and then bomb his school just days from now.
Police revealed the alleged plan Thursday, saying the potential massacre was stopped thanks to resident Chelsie Schellhas.
LaDue allegedly had amassed an arsenal in a storage facility and was heading there Tuesday when Schellhas saw him cutting through her backyard and got suspicious.
"He was walking through my backyard, didn't take the road. He had his backpack on. ... He just looked like he was homeless," said Schellhas, who was looking through her kitchen window at the time.
She called 911 and police soon arrived and found the teen in the storage locker.
"I'm grateful I wondered," she told ABC News today. "Who knows what the outcome could've been if I [wasn't] nosey."
Police said LaDue, an 11th-grader, had sketched out every detail of a massive attack. He planned to first kill his mother, father and sister and then start a fire to create a diversion for law enforcement. He then allegedly planned to head to Waseca Junior and Senior High School.
According to police, he intended to set off numerous bombs during lunch hour, kill the school resource officer as he responded to help, set fires and shoot students and staff. They said he intended to be killed by law enforcement officers.
Police said LaDue admitted placing explosive devices in a school and a park earlier this month as practice runs.
LaDue was charged Thursday with four counts of attempted first-degree murder, two counts of first-degree damage to property and six counts of possession of a bomb by someone under 18, according to the Star Tribune.
ABC News' Alex Perez and Jack Cloherty contributed to this report.