It happened around 2:30 p.m. Tuesday at the PNC Bank on the 3600 block of West Chester Pike in Newtown Square.
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Police released surveillance images showing the robber, wearing a surgical mask, walking up and threatening a teller with a gun.
The robber got away with an undisclosed amount of money.
Police did not report any injuries.
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The suspect is considered armed and dangerous.
Anyone with information should call the Newtown Township Police Department at 610-356-0602 or contact the FBI at 215-418-4000 or tips.fbi.gov.