The incident began about 3:45 a.m. outside an apartment building in the 3800 block of Cedar Avenue, according to the Riverside Police Department.
Investigators said an adult male tenant saw a man trying to break into a vehicle across the street and shouted at him to stop. The suspect fled the scene on foot and got into an awaiting vehicle, described as a white compact car.
The person behind the wheel began driving away from the building but then made a U-turn, said Officer Ryan Railsback, a police spokesman. A single shot was fired from inside the vehicle before it sped away.
Both the male victim and his girlfriend were standing inside the apartment when she was struck by the gunfire. The woman was transported to a hospital where she was pronounced dead, Railsback said.
Her identity was not immediately released.
But neighbors say the woman was known to help those in need. She worked for a church and helped hand out donations to people, according to one neighbor.
"If someone would need something she would hear about it then she would confront them if there was a way she could help them," said Evelyn Murillo of Riverside. "She was a very special person."
Investigators spent the day collecting and documenting evidence at the scene. They also canvassed the neighborhood searching for clues and surveillance video. They are also talking to other victims of car burglaries in the area.
Descriptions of the gunman and the suspects' vehicle were not available.