Rosenberg police say they need your help catching a "tough guy" who knocked out someone from behind.

Investigators said 18-year-old Alejandro Maldonado is wanted for aggravated assault after he was caught on surveillance camera running up on a man and throwing a punch.

The force of the hit knocked the man's baseball cap from his head, and you can see the victim laid out face down on the concrete after the attack.

The man police believe is Maldonado can be seen walking away from the scene as shocked onlookers scream out at the suspect.

The motive for the attack is not known.