US Attorney General John Ashcroft dispatched his top civil rights lawyer to Southern California on Wednesday to help investigate the videotaped police beating of a 16-year-old boy in nearby Inglewood, as local prosecutors began bringing witnesses before a grand jury.The officer pictured on the tape, Jeremy Morse, has been the subject of repeated complaints to the Inglewood Police Department, records show, and now faces the possibility of assault charges in the alleged attack on Donovan Jackson. In a report written Saturday night, apparently before police became aware of the existence of a videotape, Morse acknowledged punching Jackson, authorities said. He claimed to have done so only after the handcuffed boy grabbed his testicles. The report does not contain any reference to Morse picking up the boy and slamming him face first on the trunk of a police cruiser, the actmost prominent in the videotape.The national profile of the case, raised through repeated broadcasts of the videotape, was lifted still higher when Ashcroft announced that he was taking the unusual step of sending Ralph Boyd, the head of the Justice Department’s Civil Rights Division, to take a direct role in the case.‘‘The role of law enforcement officers in our society is to protect and serve the American people. The events caught on videotape in Inglewood last weekend trouble me because they raise questions about whether that law enforcement mission was being served properly in Inglewood,’’ Ashcroft said in a written statement. (LATWP)