DOVER - Police are asking for the public's help in finding a man caught on surveillance video punching a female clerk during an attempted robbery Sunday.

Investigators say the man is seen reaching over the counter and punching the woman in the face, knocking her to the ground.

Police say it happened as the man tried to rob the Multimex store in Dover. The business sells cellphones and allows people to make international calls.

Dover Police Detective Joe Camacho says police believe the man thought he was alone in the store with the clerk when he asked to see some phones from a display case. When she took them out, he punched her.

They were not alone, however. Another man is seen rushing over and startling the supect, who fled on foot.

Dover residents say they are upset by the attack but not entirely surprised, because they say crime is on the rise.

Police say the good Samaritan left the scene before they were able to speak to him and are urging him to come forward to help provide additional information.

Officers have only a vague description of the suspect and ask anyone with information to call Dover police at 973-366-2200.