The older boy died Monday afternoon at a hospital after he was taken from his friend's home in El Cajon, about 15 miles east of San Diego, said the San Diego County sheriff's department. The mother of the 10-year-old was at home at the time of the stabbing, according to the U-T San Diego.
Sheriff's officials have taken the 10-year-old into custody. Sheriff's homicide Lt. Larry Nesbit says "the case will be up to the juvenile justice system."
It remained unclear whether the boy had been booked into the juvenile detention system. The sheriff's office did not immediately return calls from The Associated Press, but said in a statement early Tuesday that the boys' identities would not be released because of their ages.
Neighbors say the 12-year-old boy lived in a nearby mobile home park but the two friends attended different schools.
A neighbor of the younger boy, 18-year-old Derek Gorton, told the newspaper he had emotional issues, and that something as small as tapping him on the shoulder the wrong way or losing at a video game could spark outrage.
"Even though he threw temper tantrums, we never thought he'd do anything like this," said Gorton.
Gorton's father, Brian Richeson, said the boy usually had verbal outbursts, but he hadn't seen him throwing things or hitting others. He praised the boy's mother, saying she had dealt with her son's problems head-on.
"She was the best mom I've ever met," Richeson told the U-T. "She knew how to take care of him if he was yelling and screaming."