Police said the gunman, who has struck mainly in Alhambra, has gotten away with several purses and wallets.
The prime targets appear to be middle-aged to elderly women from the Asian community, but men have also been victimized, as well as families with children. One man was sent to the hospital after being hit with a handgun.
Authorities say two suspects, one acting as a getaway driver, follow their victims home at night and strike when they get out of their cars.
"As they're exiting their cars, he's running up to them, demanding their money and if it's not given right away, he'll use force," said Det. Edward Rodriguez of the Alhambra Police Department.
Investigators have few leads, but witnesses have seen an older model dark-colored SUV leaving the scenes.
Robbery detectives from several police agencies met Wednesday to compare notes and conclude that there have been at least 15 victims in three different San Gabriel Valley communities since March 21.
The locations of each crime include six in Alhambra, three in Monterey Park and another five in San Gabriel, all of which occurred along Valley Boulevard between South New Avenue and Walnut Avenue.
Authorities say the suspects are becoming increasingly violent.
"In several instances, he actually became violent by punching them if they didn't give their wallets fast enough," Rodriguez said.
"What we want people to know is to be aware of their surroundings, but most of all, if they are confronted by an armed suspect, give them the property. Nothing is worth their life," added Det. Frank Duke of the Monterey Park Police Department.
Anyone with information that could help detectives is asked to contact Alhambra police at (626) 570-5136.