The robbery happened at about 3:30 p.m. The victim had just parked her car in the garage, located at Broadway and Mount Diablo Boulevard, when a man with a gun approached her, police said.
The man stood outside the woman's open car door and said something in Spanish that the woman didn't understand, Detective Steve Bertolozzi said.
Just then, another motorist drove by and the victim asked the driver to call 911, saying the man had a gun.
The man fled without taking anything, Bertolozzi said.
The suspect was described as a Hispanic man in his mid-20s with a medium build. He was about 5 feet 7 inches tall with dark hair that might have been in a ponytail. He was wearing a white tank top, dark baggy pants and white shoes, police said.
He had been seen moments earlier with a black woman in her 20s. She was about 5 feet 6 inches tall with a heavy build and dark hair. She was wearing a black top, white pants and dark shoes.
As the man fled, the woman was seen leaving the garage in a white, mid-size four-door car, police said.
Police searched the area for the suspects but didn't find them.
However, investigators have since learned that the woman returned an item at a Nordstrom store shortly before the attempted robbery and are working with the department store to identify her, Bertolozzi said.