The Maine attorney general’s office has asked a federal judge to dismiss an ex-police officer’s lawsuit against the Penobscot County district attorney on the same grounds that an appeals court threw out a similar lawsuit in 1992 against the man who was then the county’s top prosecutor. Both lawsuits involve former police officers who were either fired or put on desk duty after a district attorney refused to prosecute their cases because of concerns that their testimony could be impeached at a trial.