"This is a juvenile reacting to that bullying, trying to take matters into his own hands," said Robin Dunn, Jefferson County prosecutor.
The boy appeared in juvenile court Wednesday facing a charge of misdemeanor battery stemming from a fight at the school Monday. Police said bullying led to the fight started by the boy. No charges have been filed based on the list or weapons.
The boy's parents called police Monday evening after finding a list he made. Police found a bag containing guns, ammunition and knives. Investigators believe no one else was involved in planning the possible attack.
Keith Hammon, Rigby police chief, said a police officer was looking into reports of bullying at the school before the parents reported their concerns.
"The officer felt that there was more to what was going on with this young man and decided he needed to be evaluated," Hammon said.
The school sent a text-message alert to parents and students at about 3:50 p.m. Tuesday about the list, said Jefferson Joint School District Superintendent Ron Tolman.
"We are as prepared as anyone possibly can be for this," Tolman said. "We try to have coordination with (law enforcement) and stress to the students to let people know if they hear of things."
Parents are reacting differently to events at the school.
"Rumors had come home with my kids, and they were upset and worried," Tenessa Buttars said. "I was frustrated."
"They're OK," said parent Valerie Bowen. "It was just another normal day. They didn't have any fears."
The boy was being held at the St. Anthony Five County Detention and Youth Rehabilitation Center, where authorities said he was undergoing mental health evaluations and treatment.