It happened at approximately 11:15 p.m. on the 700 block of Adams Avenue in the city's Lawncrest neighborhood.
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An employee working at the drive-thru told police a man armed with a handgun climbed into the window and demanded the register.
The suspect took the register, which had between $100 and $150 inside, and then fled through the window.
He was last seen fleeing the scene in a black Chevy Malibu which had two other male passengers in it.
The employee told police she was not hurt during this incident.
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This theft comes just days after an attempted robbery at a Wendy's also along Adams Avenue in the Lawncrest neighborhood.
In that incident, the would-be robber attempted to climb through the drive-thru window with a metal object in his hand, but employees managed to push him out.
Police have not said whether the two cases are connected.