Jeffrey Flores, 25, appeared in a Richmond courtroom this morning to face two charges of murder with use of a knife and a special-circumstance allegation for killing multiple people, said Contra Costa County Deputy District Attorney Mary Knox said.
Flores did not enter a plea.
The special-circumstance allegation makes him eligible for the death penalty if he is convicted, Knox said.
A motive for the killings has not been determined but Flores's "significant mental illness" is believed to have played a role, said Knox.
Flores is accused of using 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.