Janitor John Luebbers, 44, was arrested at his home Wednesday after the killing of Sam LaCara, 50, principal of Louisiana Schnell Elementary School in Placerville.
No children were injured in the shooting, but police said one child might have seen the attack in the school office.
Luebbers' mother, MaryAnn Phillips, said her son was friends with LaCara, and the two had gone golfing Sunday.
She declined further comment to The Associated Press, but told The Sacramento Bee, "I really can't begin to imagine why he did it or how he did it."
She also told the newspaper her son had worked at the school for more than eight years.
The school district closed all three of its schools Thursday.
No one answered the phone at the district office to explain why Luebbers had been fired. An e-mail message sent to the superintendent was not returned.
Luebbers was expected to be arraigned Friday. El Dorado County prosecutors declined to specify the charges until then.
Residents gathered Wednesday night to mourn LaCara, a married father of three daughters. About 300 people, many of them students, former students and parents, held a candlelight vigil at the school entrance.
The curb was filled with candles, flowers and balloons.
"He's like a father to all the kids in school," Patrick Fountain told the newspaper. He said his 10-year-old daughter, Donna Zessin-Fountain, attends the school.
The school district planned to reopen schools Friday and make counselors available to students.
The Placerville Police Department also offered its condolences to the family and to staff and students of the Placerville Union School District.
The shooting prompted a lockdown of the 400-student campus about 50 miles east of Sacramento. Students were taken to the county fairground, where they were released to their parents.