Police received a call at 7:51 a.m. reporting an unknown problem in a home in the 4000 block of Marcas Street.
When police arrived, they found a woman sitting outside the house. She told officers to go inside, where they found a woman who appeared to have been shot in the head, police said.
The Contra Costa County coroner's office identified the victim Monday as 62-year-old Claudia Petretti.
Police identified the woman outside the house as 43-year-old Kimberly Petretti. She was arrested on suspicion of murder and booked into county jail in Martinez.
Police could not immediately clarify how the two women were related.
Anyone with information about the case is asked to call Detective Justin Rogers at (510) 724-8950.
People who wish to remain anonymous can call Bay Area Crime Stoppers at (800) 222-8477. Crime Stoppers offers up to $2,000 cash rewards for information leading to an arrest.
