Investigators have reportedly found the body of a woman under the floorboards of a home belonging to California shooter, Kevin Janson Neal.
The woman is believed to be his wife.
CBS reported Tehama County Assistant Sheriff Phil Johnston as saying he believes she was killed on Monday and her death sparked his rampage.
"There was a hole cut in the floor ... we're confident that he murdered her, shot her at some point on Monday, and just put her body in the hole in the floor and just covered it up," he said.
The news means a total of six people died, including Neal, who was shot and killed by police.
Nine more were injured, including seven children, Mr Johnston said.
It came as it was revealed Neal had been out on bail for stabbing a neighbour - but told his mother a day earlier that he was done feuding with his rural community, according to reports.
"Mum, it's all over now," Neal's mother said he told her in their final conversation.
"I have done everything I could do and I am fighting against everyone who lives in this area."
The New York Post reports the following day, he donned a ballistic vest, grabbed a semi-automatic rifle and two handguns and went on a 45-minute rampage across Rancho Tehama Reserve.
He killed two of his neighbours, stole their truck and drove around as he randomly picked off his targets in seven locations en route to the Rancho Tehama Elementary School.
Sarah Gonzales had just dropped off her daughter when the gunman blocked her car.
"He pretty much stopped me and shot at me three times through his windshield," Ms Gonzales said, according to CBS News.
When he stopped firing, she said, he continued toward the school, where staffers heard the gunshots and made a critical decision to lock down the building.
Mr Johnston said:"The quick action of those school officials, there is no doubt in my mind, saved countless lives,"
Neal tried to enter the school, but couldn't get in as he unleashed 30 rounds before taking off. No teachers or students were killed.
"He did shoot at a number of classrooms which resulted in the injury of at least one or two students that I'm aware of," the sheriff said.
A short time later, a patrol car rammed Neal's vehicle before officers opened fire and killed him.
A six-year-old boy, identified him as Alejandro Hernandez, was hit inside the school, the Sacramento Bee reported.
Alejandro was shot in the chest and leg, but was recovering and expected to be all right.
Mr Johnston said Neal's motive appeared to be to exact revenge on his neighbours.
"And when it got that far, (he) went on a rampage ... He was driving up and down the street shooting at passers-by," he said.
Several of his shots struck a vehicle carrying a woman and her three children.
The mum was severely injured and one of her sons was injured as well, though his wounds weren't considered life-threatening, Mr Johnston said.
"She was trying to drive herself to the hospital when I encountered her on the road," he said.
"She told me that she doesn't know this person."
One fatality was a woman who lived near the gunman and was the victim of an assault in January that ended with Neal in jail, the paper reported.
Mr Johnston said he believed Neal was slapped with a restraining order after the January arrest that would have prevented him from owning firearms, at least temporarily.
In April, the District Attorney's Office charged him with assaulting a second person, also in late January, according to Tehama County court records.
Tehama DA Gregg Cohen told the Bee that his office was prosecuting Neal on charges related to a stabbing and assault with a deadly weapon involving two of his neighbours.
Police had also visited Neal's house a day before the shootings on a domestic violence call, authorities said, but gave no details.
Neal's mother said her son recently married his long-time girlfriend.
Update your news preferences and get the latest news delivered to your inbox.