Officers acted after they found the emails written by 48-year-old Reiner Reinscheid, a professor at the University of California in Irvine. Some were to his wife and others he sent to himself, talking about using machine guns to massacre 200 pupils, the Los Angeles Times said.
Reinscheid fell into a deep depression following the suicide of his 14-year-old son, Claas Stubbe in March - he hanged himself less than 24 hours after being disciplined for a theft from a school store.
The emails shocked staff at University High School, who according to the Orange County Register, had no idea just how angry Reinscheid was.
Yet in emails sent in April to his wife and to himself, Reinscheid threatened to kill the school's assistant principal, rape female staff, shoot hundreds of students and then burn the school down, the paper said.
"I need a gun, many guns, and then I have the ride of my life," Reinscheid wrote in an email to himself.
"I will give myself a wonderful ending and be with Claas very soon. I like this plan, finally a good idea."
The email also said that he was drinking a second bottle of wine that evening and had taken medication that was keeping him awake.
Officers found the emails after Reinscheid was arrested on July 24 for trying to start a fire in a park, near where his son had killed himself. It also emerged that he had made a new will on the same day.
He had, the Orange County Register said, been charged with five counts of arson and one count of attempted arson. Three of the fires were at the vice principal's house. Two were at the school itself.
He was released the same day on bail, but rearrested on July 27 after police found the emails while examining his mobile phone.
Prosecutors submitted the emails in court on Tuesday as part of a motion that he be denied bail, the LA Times said. The emails were not enough in themselves to warrant his arrest as they were not sent to any potential victims.