The incident happened around 3 a.m. at Fair Price Market in Westernport.
Police say a man entered the store armed with a gun and demanded money before fleeing the scene on foot with an undisclosed amount of cash.
The suspect is described as a white male, about 5'3" to 5'4" with a thin build. He was last seen wearing a heavy, dark coat and toboggan. Authorities say his face was covered with a black and white bandanna.
A K-9 unit did assist in the investigation.
Anyone with information is asked to call the Allegany County Sheriff's Office at (301)-777-1000.
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