For two weeks after the death of a Staten Island man in police custody, frustration simmered as the city awaited the results of the autopsy. The mayor offered sympathy. The police promised better training. But now, with the medical examiner’s conclusion that the death was a homicide, by chokehold and chest compression, the investigation — and most significantly, the question of whether to prosecute any police officers — rests in the hands of the Staten Island district attorney’s office.

It is a decision fraught with legal and political complications, all the more so because Staten Island is home to many police officers and, more than any other borough, is seen as sympathetic to law enforcement.

Indeed, even as the district attorney, Daniel M. Donovan Jr., continues to investigate the police officers’ actions in the death of the man, Eric Garner, the Rev. Al Sharpton has called for a Justice Department investigation and went to Washington on Monday to urge federal action.

The two largest police unions, which have vocally defended the officers who were trying to arrest Mr. Garner, planned a news conference for Tuesday morning to lash out at “police haters.” On Tuesday afternoon, the new chairman of the Civilian Complaint Review Board, Richard D. Emery, is holding an unusual special meeting to address, among other topics, his investigation into hundreds of chokehold complaints against officers in recent years.