It happened around 5:30 a.m. Monday morning at an Exxon Station on Reliance Road in Middletown.
According to witnesses, a man entered the store, pulled out a folding utility knife and threatened the clerk. He was able to get away with an amount of cash before running from the scene.
"It is right off the interstate, so they'd be able to jump off and commit some sort of crime like this, and jump back on. But for the general area of Middletown, its very unusual," said Capt. Donald Land, Frederick County Sheriff's Office.
Police are currently searching for the suspect. He's described as a white man, in his early to mid 20's.
Anyone with information is asked to call the sheriff's office at (540)-662-6162.
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