Horsley said the 11-year-old boy was booked on suspicion of bringing a dangerous weapon onto school grounds and aggravated assault for allegedly pointing the weapon at students on the playground Monday.
Horsley said the ongoing investigation is being handled by the Granite District Police Department.
In addition to being taken into custody, the boy faces what Horsley said was an unspecified but "lengthy" suspension. That suspension applies to all schools and programs in the district; the boy will not just be able to be transferred to a different campus.
"This is an egregious violation of our Safe School policies. We take this very seriously," Horsley said, adding that the boy's family "has been very cooperative and helpful to our investigation."
Horsley said the boy had obtained the gun from his home, where a relative had apparently left it during a visit last week.
Two of the boy's classmates told their teacher about the gun at 3 p.m. Monday, about 45 minutes before the end of the school day. The teacher immediately took the boy to the principal's office. The unloaded gun, along with an unspecified number of bullets, were found inside the boy's backpack, Horsley said.
No one was injured. The school was not placed on lockdown.
"The way this event played out, considering the potential outcomes, we are very pleased with the actions taken by the students who reported it and also the teacher and administrators," Horsley said. "It could not have gone better in terms of immediately alleviating the threat and getting the student and weapon into custody."
Crisis counselors were at West Kearns Elementary on Tuesday to try to calm any remaining anxiety as well as to help teachers "quell any rumors" about what happened, and to answer questions about school security that students or staff may have.
Gun threats were hoaxes
Authorities questioned several students, searched a Weber County school Tuesday morning for weapons and temporarily added sheriff's deputies to campus security after a text message circulated claiming that someone was going to bring a gun to class.
Weber County sheriff's Lt. Mark Lowther said as of Tuesday afternoon that the rumor involving Bonneville High School, which spread rapidly through social media postings, appeared unfounded.
Weber County School District spokesman Nate Taggart also said the gun report appear to be a hoax, and the person or persons responsible for starting it could face criminal charges.
Weber County's threat marked the second apparent text hoax in as many days.
On Monday, authorities responded to Box Elder High School after the sheriff's office received a text warning that a student may have brought a gun to school, KUTV reported.
Authorities later determined there was no basis for that threat.
The Salt Lake Tribune