Bank of America branches were targeted in four of the holdups.
Witnesses described the robbers as four black men, armed with guns. During the robberies, employees have been assaulted inside the bank.
The first robbery took place in March of 2009 at a U.S. Bank branch in Thousand Oaks. The most recent robbery occurred on January 21 in the city of Westminster.
The robbers were nicknamed the "20 Questions Bandits" due to witness reports that they asked several questions during the initial robberies.
Anyone with information was urged to call the FBI at 1-888-CANT-HIDE (1-888-226-8443).