A possible motive for the killings discovered Monday was not disclosed, and police did not say if the victims might have been related.
The deaths brought to nine the number of people killed in a 51-hour period in Stockton, an agricultural city in Northern California.
Police investigating the murder-suicide said they initially found a 64-year-old woman shot to death at her home near the University of the Pacific.
A minivan spotted leaving the scene was located minutes later at an upscale retirement home, where police found an 88-year-old woman and a 45-year-old man - the suspected gunman - dead from gunshot wounds inside a room.
"We are looking at the aspect of did he take his own life?" police spokesman Joe Silva said.
A 57-year-old woman was then found in the back of the minivan.
Police Chief Eric Jones went to the retirement home and joined a swarm of homicide detectives and officers behind yellow crime scene tape.
The killings occurred during a violent three-day stretch in Stockton, which has now seen 59 homicides so far this year - one more than the 58 in all of 2011.
"I've never seen this many homicides in such a short time span," Silva said.
Also Monday, a 31-year-old man and 23-year-old were shot and killed in south Stockton in a possible gang-related shooting, police said.
The string of killings began Saturday with the brazen lunchtime shooting of a man outside a fast-food restaurant. Two other fatal shootings occurred late Saturday and early Sunday.