LEHI — Police arrested a Skyridge High School student after they say he threatened to commit a mass shooting.

The 18-year-old Lehi student was booked into the Utah County Jail Thursday for investigation of making a terroristic threat.

Police say the student went to a new therapist for the first time on Thursday and "unloaded" on her. The therapist later called police to say he had threatened to "shoot up his school," a police affidavit filed in 4th District Court states.

During the course of the 45-minute conversation with the therapist, the student said he "sees himself as a vigilante" and talked about "his desire of shooting up his school and murdering a person that hurt his friend" as well as his "manic episodes of not being able to control his anger," the affidavit states.

The 18-year-old would not "disclose names, when he would carry out his plan(s), or details of the plans. He only explained that he fantasizes over these murderous thoughts constantly and he spent 3-plus hours last night (Wednesday) planning it out," the report states.

The therapist said while she did not believe a threat was "imminent," she wanted to make law enforcement aware of the situation.

Police said they felt the threats "were very real and credible," according to the affidavit. Lehi officers went to look for him at Skyridge High, where they discovered that a student assembly was underway in the auditorium. They then learned that the student was at his house and went there.

When interviewed by police, he "opened up" to investigators, "releasing frustration, odd responses and several bone-chilling responses to our questions," the affidavit states. "After speaking with (him), all officers on scene felt the next step of (his) plan was execution — execution on Skyridge High School and specific people."

The student told police that he would only hurt the "bad guys" and not the "good guys" such as the school resource officer.

In explaining his plan, the student "considered response time from police or the SWAT team and where the 'best vantage point would be,"' police wrote. "Based on (his) admission of 'desire to cause mass destruction and not just kill select a few,' he was arrested."

"There was enough there that the officers felt it was a credible threat in order to make the arrest," Lehi Police Sgt. Damon Faught said Friday.

The student does not have a history of violence, he said.

Skyridge High officials informed parents of the arrest in a prepared statement on Friday.

"Our students, faculty, and staff are all safe. We are so grateful to the Lehi Police Department for their quick resolution of the situation and for their professionalism in helping ensure the safety of our school community."

Contributing: Ashley Imlay