Police said the suspect walked into the Chase bank at 12555 Valley View St. in Garden Grove with a device that appeared to be an explosive, forced an employee into the safe and escaped with an undisclosed amount of money.
As he left, he allegedly told the employee he had left the device behind.
Police evacuated the bank and surrounding businesses, but a sheriff's bomb squad did not find anything.
The suspect is described as a white man in his 40s with dark hair, a blue baseball cap, a beige long-sleeve shirt and jeans.
Witnesses should call investigators at (714) 741-5800.