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The 16-year-old boy who brought a loaded gun to Fremont High School on Monday was planning to shoot a former girlfriend and open fire on the rest of the school, police officials said Tuesday.

Weber County Sheriff's Lt. Lane Findlay said the teen was upset about "supposed rumors" being spread about him at school.

"He thought there was somebody talking about him," Findlay said. "So he was going to bring the gun to school."

Findlay added: "There was a suspicion this person [talking about him] was his former girlfriend, and if so, he was going to shoot her."

The girl and her family were notified so she was aware of the situation, Findlay said.

Findlay said the teen stole the 9mm semiautomatic handgun — which can hold up to 11 rounds — from a family member.

A female student spotted the gun shortly after noon Monday and alerted a school resource officer, who "detained the suspect without incident," according to sheriff's office authorities. The gun was found concealed in the boy's waistband.

Findlay said that authorities "applaud the courageous effort" of the student who reported seeing the gun.

The teen, who was not publicly identified because he is a juvenile, was being held at a juvenile-detention facility on suspicion of three firearms-related misdemeanor charges.

The school, located at 1900 N. 4700 West in Plain City, was evacuated for about four hours Monday as 70 law enforcement officers searched the school one classroom at a time for any other threats, and also patted down each student before allowing them to leave. Findlay said this was done because when the teen was asked by police if anyone else was involved, he "made some ambiguous comments," saying he "didn't know."

Findlay said 1,700 students had to be evacuated, adding that the students were respectful and well-behaved as the officers completed their search.

In addition to the Weber County Sheriff's Office, officers from Pleasant View, Harrisville, the U.S. Forest Service, the Utah Highway Patrol, Ogden, and the Ogden Metro SWAT team responded to the high school Monday.

"When the call came in, a student with a gun, there was just massive response from all law enforcement in Weber County," Findlay said. Classes were proceeding as scheduled Tuesday, Findlay said, but some law enforcement officers were still on campus to help students feel safe about returning to school.

Twitter: @jm_miller