About $5,000 was taken in the robbery, which occurred at about 9:15 a.m. in the County Shopper mall at 3130 Alpine Road, authorities said.
The bank's surveillance camera shows a man with brown hair and a mustache and wearing a yellow bicycle jacket and a white baseball cap.
The man told the bank teller "give me all your cash," gave her 60 seconds to do that, and added that he was armed, Lt. Ray Lunny of the San Mateo County Sheriff's Office said.
The teller emptied the cash drawer, handed it over, and the suspect put the money into a blue bag he was carrying, and fled, deputies said.
The suspect is described as a white man, 40 to 45 years old, and of middle height and weight.
Sheriff's deputies are coordinating with the FBI in the search.
Anyone with relevant information is asked to call 415-553-7400 to make confidential contact with the FBI, or the Sheriff's Office at 363-4051. The anonymous tip line is 800-547-2700.
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