CHICAGO (CBS)–An armed robber wearing a clown mask robbed a South Loop Dunkin Donuts early Thursday, according to police.

An employee was working alone in the store on the 1500 block of South Michigan Avenue when the masked gunman, wearing a green coat and green pants, walked in an demanded cash, police said.

The suspect told the cashier to open the register, and at  that point the robber grabbed the register and ran out the back door in an unknown direction, police said.

Area Central Detectives are investigating.