Police were called to the 800 block of Sullivan Street around 8:30 a.m. The caller said the baby was not breathing.
The child had been left in the care of its grandfather and 11-year-old sibling. The baby's mother was not home at the time of death but did return home Sunday morning. The mother and other family members are being questioned.
The Santa Ana Police Department is conducting an investigation.
"We're investigating whether it's a SIDS death, an accident. There appears to be no signs of crime or trauma to the child," said Santa Ana Police Cpl. Anthony Bertagna. "At this point we are just trying to figure out what happened."
Children Services is also conducting an investigation, which is routine during child death incidents.
An autopsy will be performed Monday or Tuesday to determine cause of death.