ATHENS — The photo splashed on the cover of a Greek newspaper this weekend shocked a nation: Pavlos Fyssas, a Greek rapper whose music inveighed against far-right groups, lay dying in a pool of his own blood as his girlfriend cradled him in her arms, moments after he was stabbed in the heart.

The suspect has been linked to Greece’s neo-fascist Golden Dawn party. Almost as chilling are accusations by some witnesses that a squad of police officers stood by as a group of burly, black-clad party members chased Mr. Fyssas down. A police spokesman denied that account, saying officers arrived right after the stabbing, in a gritty Athens suburb last Wednesday, and promptly arrested the suspect.

The killing of Mr. Fyssas has spurred the government to begin a risky crackdown on Golden Dawn, opening its first investigation into whether the police forces are infiltrated by sympathizers or members of the group, one of the most violent rightist organizations in Europe.

On Tuesday, officers raided three police stations on the outskirts of Athens. The sweep came a day after the government replaced seven senior police officials — including the chiefs of special forces, internal security, organized crime and the explosives unit — to ensure the investigation would take place with “absolute objectivity.” In addition, two top members of the Greek police force resigned abruptly Monday, citing “personal reasons.”