An officer repeatedly yanked on the dreadlocks of a handcuffed black man who was arrested after a dangerous high-speed chase in Iowa, then twice hit him on the back of the head.Waterloo officer Adam Wittmayer briefly assaulting 24-year-old Montavis Keller and calling him names after the April 19 chase.A state prosecutor concluded in June that no criminal charges were warranted against Wittmayer because jurors would understand why he was mad.Keller is charged with nearly hitting Wittmayer with his car at the end of the chase after Wittmayer put down tire-puncturing sticks to disable it.Waterloo Police Chief Daniel Trelka says Wittmayer was disciplined and that he doesn't condone the officer's behavior.