Cedric Anderson shot and killed his estranged wife Karen Smith at North Park Elementary School on April 10. The gunfire also claimed the life of a student, 8-year-old Jonathan Martinez, who was standing near his teacher. Anderson then shot and killed himself.
A second student, 9-year-old Nolan Brandy, was also struck and survived the shooting.
Now for the first time, we're getting a look at what happened moments before the shooting.
Video shows San Bernardino school gunman moments before murder-suicide
The school surveillance video first shows Anderson trying to enter the building through the front doors, before realizing they are locked.
He then walks in to the school front office, briefly says something to the office staff and signs in. Police said he told the staff he was there to drop something off for his wife.
He gives no visible sign of emotional disturbance as he talks to the staff and then walks into the school.
Security has since been changed at North Park.
Visitors are no longer allowed beyond the main office and video screens have been installed at the secretary's desk to help decide whether or not someone should be buzzed into the building.