The bandit has hit four Bank of America branches in Anaheim, Fullerton and Garden Grove from October through December.
Police said the suspect barges into banks with a semiautomatic weapon and yells that it's a robbery, demanding cash from the tellers.
He was described as possibly being in his early 40s, 5 feet 8 inches to 6 feet tall and about 180 pounds, with a muscular build.
He has carried a black plastic shopping bag and worn reading glasses, and was further described by witnesses as well-spoken, with non-accented English and a low voice. Witnesses have also described acne scars on his jawline and a pointy nose, Eimiller said.
The reward of up to $5,000 was offered by Bank of America, Eimiller said.
Anyone with information is urged to the FBI at (714) 542-8825, or a number set up by the robbery task force at (888) 226-8443.
City News Service contributed to this story.
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