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A Utah teen accused of attacking five classmates in an Orem high school locker room told police that he had been suicidal for years, according to a newly unsealed search warrant affidavit.

The 16-year-old boy is charged in 4th District Juvenile Court with five counts of attempted murder, a first-degree felony, along with other misdemeanor charges, accused in a Nov. 15 attack that left five students with serious injuries.

The boy brought five knives and a wooden staff with him to Mountain View High School to be used in the assault, according to a search warrant affidavit that was unsealed on Monday.

After the attack, the boy told police that he had planned the attack days before, and used his mother's laptop, his iPad and his cell phone to look up YouTube videos to help him plan.

"[The teen] stated he has had suicidal and harmful thoughts since a very young age," a police detective wrote in the affidavit, adding that the boy had left a suicide note at his Orem home for his parents to find.

The boy allegedly attacked the first student with the wooden staff, which broke when he struck his classmate. He then stabbed four other students before stabbing himself, police have said.

"There is no apparent motive or cause for the stabbing," the detective wrote. "[He] was reported to be 'elated' as he was attacking different students with the knife."

The victims appeared to be random, police say, and were targeted based on their proximity to the teen.

Prosecutors have not yet decided whether to seek to have the teen charged as an adult, Deputy Utah County Attorney Sam Pead said after the boy's first court appearance last week.

Juvenile Judge Douglas Nielsen has ordered a competency evaluation for the boy, who is scheduled to be in court again on Feb. 14.

In the meantime, the boy will continue to be held in a juvenile detention facility.

The Salt Lake Tribune does not generally identify juveniles charged with crimes unless they have been certified to stand trial in adult court.

jmiller@sltrib.com

Twitter: @jm_miller