A note handed to a teacher at Canyon Springs High School in Moreno Valley led to the student's arrest Monday on suspicion of making terrorist threats.
Authorities identified the student as 18-year-old Fernando Hernandez. He was detained after the teacher reported the threat to a School Resource Officer.
"It was a note that contained a very specific statement about a desire to shoot this particular teacher," Moreno Valley Police Chief Joel Ontiveros said.
Deputies confiscated a bolt-action rifle with a "small amount of ammunition" at the home of the student's family. The rifle, which was legally purchased, had a security lock, according to the Riverside County Sheriff's Department.
At the campus on Tuesday, parents and students said they were shocked. It seemed only a few students knew the suspect.
The school sent a message to parents stating that the school had not been on lockdown and there were no other incidents on campus.
"We continue to be proactive regarding the safety of our students and staff," the statement read.
The school had planned a 1 p.m. vigil on campus for the victims of last week's shooting at a Newtown, Conn., elementary school that killed 26.
Police in Moreno Valley said they were scrutinizing this situation in light of a rash of violent incidents on school campuses, including the Newtown shootings.
A court date for Hernandez was scheduled for Wednesday, according to Riverside County jail records.