Kevin Janson Neal killed his wife and hid her body in his home before he went on a gun rampage and fatally shot four people, California police have said.
"We believe that's probably what started this whole event," Tehama County Assistant Sheriff Phil Johnston told reporters on Wednesday.
Neal, 43, fired into an elementary school in Rancho Tehama, but was stopped from entering by teachers.
Hi wife's body was found hidden under floorboards, police say.
"This individual was literally going up and down the road and shooting at random structures", the assistant sheriff said.
"So it is possible that you may have a neighbour or somebody that could be injured or hurt that we are not aware of," he added, calling on residents to check on their neighbours.
Officials have not yet given a motive for the attack.
Authorities praised teachers' for locking down the school campus after hearing gunshots, saying their actions were "monumental" in saving "countless" lives.
Fourteen people were injured in the rampage, including seven children, police say.