CHARLESTOWN, Mass. -- Police are searching for a serial bank robber who they said has held up six stores in Charlestown, Cambridge, Somerville and Boston's Jamaica Plain neighborhood in the last week.

Police released surveillance video of the suspect Monday and several store owners talked about their encounters with the armed thief.

"The guy ... put up a gun and asked him to open the registers," said 7-11 store clerk Peter Wadza.

Investigators said the robbery spree began at a Tedeschi's in Cambridge last Tuesday and several heists followed in rapid succession, with the thief following a pattern, hitting convenience stores and first acting like he's a customer buying something.

"The MO is to buy something, to get the cash register to get open and that's when he pulls out the gun," said Boston Police Superintendent Rafael Ruiz.

"I give a lot of money, all of bills," said one store clerk, describing how the suspect took the bills and stuffed them in his pocket before fleeing.

A Dunkin' Donuts in Charlestown was where the most recent holdup occurred.

Police said the robber is not doing a lot to conceal himself as he was not wearing a hat or mask.