Police are investigating a robbery early Sunday at the 7-Eleven at 13639 Dumfries Road in mid-Prince William County.
A masked man walked in just before 3 a.m. and brandished a firearm, forcing the employee behind the counter and demanding money from the registers.
The employee was then forced into a back room before the robber fled the business on foot, Prince William County police spokeswoman Renee Carr said.
A police K-9 and the Fairfax County police helicopter searched the area but didn't find anyone.
The robber was black, about 18 to 20 years old and wore a white surgical-style mask, a black jacket, a black hooded sweatshirt, white gloves, black pants, and black shoes with white soles.
Anyone with information is asked to call police at 703-792-6500.