It happened at the Fidelity Bank on North Delaware Drive in Upper Mount Bethel on July 11, troopers said in a release.
The suspect approached a teller at 11:48 a.m. and handed over a note demanding money before making off with an undisclosed amount of cash, according to authorities.
He is described as a "Hispanic male" standing 5-foot-5, and was last seen wearing a black hoodie and a blue surgical mask, they added.
Anyone with information is asked to call the state police Belfast Station's Criminal Investigation Unit at 610-759-6106.
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