"As a result of the initial search of the suspect's residence, several cans of gasoline, presumably Molotov cocktails, 12 firearms and approximately 22,000 different types of ammunition were found," the statement said on Twitter.
As noted, the motives of the attacker have not yet been established, an investigation is underway.
According to preliminary data, the suspect acted alone.
"It is clear that this was a planned attack, and the suspect was ready to use firearms to kill as many people as possible ..." - added in the sheriff's office.
Earlier it was reported that as a result of the shooting in San Jose, eight people were killed, several more were injured.
The death toll later increased to nine.