The autopsy revealed multiple blunt injuries to the head, extremities and torso, said Danville Sgt. Troy Craig.
The District Attorney is looking to try him as an adult.
"At this time we are doing the investigation," Wenzel said, at a press conference Monday afternoon. "This is very tragic. It's something rare. We are still looking into all aspects of (the incident)."
The police found the dead woman in a bedroom in the home in the Shadowhawk community, according to reports, and initiated a homicide investigation.
Neighbor Steve Niforoushan recalled police banging on his door at 2 a.m. and asking him about the boy.
At 4 a.m., he heard helicopters hovering over his home.
"Every cop in Danville was here last night," he said Monday.
He noted that, before the incident, the teenager had knocked on his door several times and told him: "I have a problem in my head."
Mantas had been recently divorced from the boy's father; she also had two daughters but they were apparently not present when the murder took place. Mantas worked as a teacher's aide in special education at Tassajara Hills Elementary School, said Terry Koehne, spokesman for the San Ramon Valley Unified School District.
"We are shocked and saddened by this incident," he said.
Koehne declined to comment on what high school the suspect attended, although students at the press conference said he had formerly gone to Monte Vista High.
Teenager Taylor Coyle said she had hung around with the suspect a few times.
"It's a big shock to everyone here," she said.
"He was a nice, outgoing kid," she added.
Wenzel said police are still doing an investigation to find out more details.
The San Francisco Chronicle reported that the victim, who had custody of her son, had accused her husband in divorce documents of being violent and abusive and "extremely hurtful" to the son and for increasing the teenager's "feelings of anger and sadness."
This story contains 403 words.
If you are a paid subscriber, check to make sure you have logged in. Otherwise our system cannot recognize you as having full free access to our site.
If you are a paid print subscriber and haven't yet set up an online account, click here to get your online account activated.