TULSA, Oklahoma - Police are asking for the public's help to find a serial robber.

Officers say he struck again Monday evening using a sawed-off shotgun to rob a Dollar General store near 4th and Lewis.

Police say this is the third armed robbery of a Dollar General store in the past three days. The other two robberies happened Saturday, January 6th. One of those stores was at 28th Street North and Peoria.

In the Monday night's robbery, police say the robber took money from the store and from a customer who was checking out. After getting the cash, the man ran out of the store.

Police say no one inside the store was hurt, but a customer who talked with News On 6 says he was shaken up.

"I just hope if he sees this that he knows that he's destroying people's lives just to get a little money. Why doesn't he do the right thing and turn himself in,” said that customer.

Police Captain Mike Williams says officers used dogs to try to find the robber, but he got away.

If you know anything that can help police catch this robber, call Crime Stoppers at 918-596-COPS.