Before entering his plea, Mr. Esposito signed a cooperation agreement with the Manhattan district attorney’s office under which he must testify against the other defendants in the case. Under the terms of the agreement, if his testimony is satisfactory, he will be allowed to plead guilty to a lesser charge of second-degree grand larceny and prosecutors will recommend a sentence of one and a half to four and a half years in prison. Justice FitzGerald warned Mr. Esposito that he would face a much stiffer sentence — eight and one-third to 25 years in prison — if he failed to live up to the agreement.

With his plea, Mr. Esposito, who was a police officer from 1973 to 1990, joined 87 other defendants who have pleaded guilty in the case after being swept up in mass arrests in January and February. Most of the defendants have been sentenced to probation in return for a promise to repay the money they received from Social Security.

Outside court, Mr. Esposito’s lawyer, Brian Griffin, played down his client’s part in the scheme. “Although neither the architect, nor the mastermind, Mr. Esposito acknowledges that, in his role, his actions crossed both an ethical and legal line, and for that he takes responsibility,” Mr. Griffin said.

Court papers, however, described Mr. Esposito’s role as pivotal. He recruited many of the people who applied for the benefits and introduced them to three others accused of helping to run the operation — John Minerva, 62, another former New York City police officer; Thomas Hale, 90, a pension consultant; and Raymond Lavallee, 84, a Long Island lawyer — prosecutors said in a bail letter and other filings. All three have pleaded not guilty and are awaiting trial.

Mr. Esposito and Mr. Hale, court papers say, referred most of the applicants to two psychiatrists for treatment and to establish a year’s worth of medical records. On several telephone calls recorded by the authorities, Mr. Esposito was captured coaching applicants on how to mimic the symptoms of depression and post-traumatic stress when being examined by doctors.