The incident, which was captured on surveillance video, occurred about 11:25 p.m. at the convenience store at 4500 Holt Avenue, according to the Montclair Police Department.
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The footage shows the suspect entering the store, walking behind the counter and pointing a black semiautomatic gun at the cashier. The robber then demands the clerk open the cash register.
The employee was unable to open the register, investigators said. The suspect then took an undisclosed amount of cash from the clerk's hand and struck him in the back of the head with the gun.
The cashier hit his head on the counter and fell to the ground as the robber fled on foot.
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The man being sought is described as 20-30 years old, 6 feet to 6-feet-2-inches tall, with a slim build, wearing a gray long-sleeve shirt, dark pants, a blue and red baseball hat, black face covering and black shoes.
The victim suffered two small cuts and declined medical attention at the scene, authorities said.
Anyone with information about the crime was urged to call Montclair police at (909) 621-4771.