The robber entered the bank at about 9:08 a.m., placed a black bag on the counter and demanded cash, police said.
"(The robber) said give me hundreds, hundreds, hundreds, and passes a note to the teller who passes over an undetermined amount of cash," said Sgt. Diedrich Hohn.
The robber then fled the First County Bank, which is located at 1042 High Ridge Road.
Police didn't know whether the robber was a man or woman because the individual was "very hoodied up," Hohn said, and was also wearing a bandanna across their face.
No one was injured, although the teller was very shaken up, Hohn said.
Police were investigating whether the robber had accomplices because one witness said a vehicle with two white males went through a red light at the intersection where the bank is located, Hohn said.
Police are waiting for surveillance video from the bank to help them get a better description of the suspect, he said.
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