But several months ago, the officials said, Mr. Kaplan's resolve weakened. In an effort to win a sentence reduction, the authorities said, Mr. Kaplan began to cooperate with a team of federal and state prosecutors and agents from the federal Drug Enforcement Administration and the F.B.I. who had been trying to breathe new life into the case against the detectives.

The development, several of the officials said, made Mr. Eppolito and Mr. Caracappa's indictment possible. The two men were arrested on March 9 in Las Vegas, where they had retired and where they owned homes across the street from each other in a gated community.

Now, Mr. Kaplan appears destined to play one more role: key witness against two retired police detectives who were charged by federal prosecutors last week with some of the most spectacular acts of wrongdoing by New York City law enforcement officers ever - killing mobsters on behalf of mobsters.

It was Mr. Kaplan, law enforcement officials said, who served as the intermediary between the two rogue detectives and Anthony Casso, the onetime Luchese underboss who, prosecutors say, paid the detectives for their help in killing half a dozen or more people in the late 1