FBI officials said an elderly, thin, grey-haired man nicknamed the "Geezer Bandit" is responsible for holding up five San Diego-area banks since summer.
Investigators say the man appears to be in his 70s.
Officials say that in the most recent robbery on Monday, he approached a Bank of America teller in La Jolla, displayed a handgun and asked for cash. He fled on foot.
FBI spokesman Darrell Foxworth said investigators believe the man is also responsible for robbing four other San Diego County banks since August 28.
Law enforcement officials are offering 16,000 US dollars in rewards for information that leads to his arrest and conviction.