Jeffrey Flores, 25, appeared in a Richmond courtroom Wednesday morning to face two charges of murder with use of a knife and a special-circumstance allegation for killing multiple people, Contra Costa County Deputy District Attorney Mary Knox said. He did not enter a plea.
The special-circumstance allegation makes him eligible for the death penalty if he is convicted, she said.
Knox said a motive for the killings has not been determined but that Flores's "significant mental illness" is believed to have played a role.
Flores allegedly used a butcher knife to fatally stab his grandmother, 76-year-old Nona Jean Flores, and her son, Tony Flores, 54, at their home in the 1100 block of Tara Hills Drive, the prosecutor said.
Officers were dispatched to the two-story house at 2:30 a.m. after someone called to report an unspecified problem at the home, according to police.
Police found the victims in separate bedrooms. Both were pronounced dead at the scene, and Flores was arrested on suspicion of murder.
A woman believed to be Flores' sister was also at the house when officers arrived, along with three children under 10 years old, police said.
Flores is set to return to court in Richmond for a possible plea entry next Monday.