Investigators believe the attack happened during the day when the woman was alone in her home at the Park Regency Apartments in the 3100 block of Oak Road.
The suspect is believed to have entered through the front door, which was unlocked, then raped the victim and fled, Lee said.
The attack was reported to the sheriff's office at about 7 p.m.
The suspect is described as a black man with a dark complexion. He was between 20 and 30 years old and about 6 feet to 6 feet 2 inches tall, with a slender build. He was wearing dark clothing.
Investigators believe the same man may have been involved in an earlier incident at the same apartment complex on June 30 at about 1:15 a.m., Lee said.
In that incident, a man knocked on the front door of an apartment and asked a woman for a glass of water. When the woman went to get him some water, the man entered the apartment and covered the victim's mouth with his hand.
The woman was able to push the man away and scream, which alerted her boyfriend and the suspect fled.
Anyone with information about the case is asked to call sheriff's detectives at (925) 313-2625.