San Francisco police officers took 45-year-old Oakland resident Matthew Eisenhower into custody around 9:30 p.m. on Friday, San Francisco police spokesman Albie Esparza said.
He was arrested on suspicion of stalking and making criminal threats. He was booked into jail and is being held in lieu of $1 million bail.
Eisenhower's arrest came just hours after school let out for the day at Peralta Elementary School at 460 63rd St. in North Oakland.
Police and school officials believe Eisenhower sent letters to a mother of a student at the school. In the letters he indicated that he would harm the student's family and that he knew where the child attended school.
The threats caused school officials to take safety precautions on Friday, including the addition of a police officer on campus and the closure of all but one gate at the school.
Police in Oakland and San Francisco were notified of the threats, since the mother received the threats while she was at work in San Francisco.