If Betty Hydell had not turned on the television that afternoon in 1992, she might never have learned the stranger’s name. But there on the Sally Jessy Raphael show was the bruiser who had knocked on her door six years earlier looking for her son. He had come asking for 28-year-old Jimmy on the day he disappeared—and, she had no doubt, was murdered. Only, now that she knew the man’s name, justice, she was convinced, was impossible. He was beyond the law.

Six years later, she lost another son. Frank, 31, the younger brother, was found lying between two parked cars in front of a Staten Island strip club with three bullets pumped into his head and chest. Now she needed to talk; and slowly, despite her anxieties, she was growing ready.

Finally, in the fall of 2003, say those who participated in the case, Betty Hydell, then 65, shared her long-held secret. It was a secret that would have momentous consequences. This single name resurrected old suspicions and set in motion a covert 18-month investigation that led a team of retired New York cops and Drug Enforcement Administration agents back to the bloody gangland wars of previous decades, and had them hunting through seemingly ice-cold cases and unsolved murders. And at the end of their long investigative journey they uncovered what law-enforcement officials are calling “the worst case of police corruption in the history of New York.”

In March, two retired New York City Police Department detectives, Louis Eppolito, 56, and Stephen Caracappa, 63, were charged with working for the Mob. Even as detailed in the careful sentences of the 27-page federal indictment, the alleged betrayal, which began in the mid-1980s, was both riveting and complete. On the surface, as many of their astonished fellow cops were quick to point out, the pair had been exemplary police officers. Eppolito, big, beefy, and loud, had been a tough street cop, a head-banger who bragged that he had been in eight shoot-outs and had survived to become the N.Y.P.D.’s 11th-most-decorated officer. Caracappa was more cerebral, quiet and ruminative, a cool dandy in the trim black suits he had made in Hong Kong. He, too, had put together an impressive two-decade career, serving on the elite Major Case Squad and winning a promotion to detective first grade.

Yet, according to the indictment, while they had been building their careers and passing themselves off as gung-ho cops, they had been taking orders from the Mob. In dozens of cases, they allegedly gave the Mafia the edge, allowing wiseguys to get away with murder—literally. They revealed the names of individuals who were cooperating with the government, and as a result three informants were killed and one was severely wounded. They shared information about ongoing investigations and pending indictments with the Lucchese crime family, one of New York’s five major Mafia clans. But most shocking of all, and unprecedented in the history of the N.Y.P.D., they had acted as paid killers. The two detectives were charged with taking part in at least eight Mob hits—including one where they were the shooters. (The body of a ninth suspected victim was discovered after the indictment.)

Incredibly, allegations about the two detectives were first made more than a decade ago. But officials were never able to get the evidence they needed for an indictment.

“We were only able to make this case,” says one of the key investigators on the task force, “because after years of stonewalling we succeeded in getting the man who paid Eppolito and Caracappa to talk.”

However, unknown to the task force, their star witness had long been an informant for the F.B.I. And according to dismayed law-enforcement officials, if the F.B.I. had shared this information with the N.Y.P.D., the two rogue detectives could have been prosecuted years ago.