Plea negotiations are underway in the case of two NYPD cops accused of running a protection racket among Queens karaoke clubs as part of an investigation that has led to the questioning of 21 other officers, according to lawyers familiar with the cases.

Former Lt. Robert Sung, 50, and Det. Yatu Yam, 35, were charged in criminal complaints in December with taking bribes from some Flushing karaoke club owners in exchange for tipoffs about upcoming drug raids. They also faced charges of official misconduct, according to state court records.

Though neither Marvyn Kornberg, who represents Sung, nor Jeffrey Licthman, the attorney for Yam, would comment in detail, they said plea negotiations were underway with Queens prosecutors in an effort to resolve the cases before trial. A spokesman for the Queens district attorney’s office declined to comment on the case Friday.

Sung was fired earlier this year for reasons unrelated to the corruption allegations, Kornberg said. Yam was originally suspended without pay but after 30 days was restored to full-pay status as required by civil service laws, an NYPD spokesman said.

The continuing investigation into the 109th Precinct has led police internal affairs investigators to question nearly two dozen cops, including three over possible bribery allegations, according to a source familiar with the probe. Other officers have been questioned about failing to tell internal affairs of possible corruption, failing to follow police procedures and unauthorized use of department computers, the source said.

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No additional officers have been charged in the case. However, a number of cops have been transferred out of the 109th Precinct because of the investigation, according to police.

The original complaints in the criminal case indicated some unnamed officers questioned the actions of Sung and Yam but apparently didn’t communicate their complaints to internal affairs. Failure of cops to report such suspicions is grounds for disciplinary action.

The probe into the 109th Precinct is unrelated to the corruption investigation by the NYPD and the FBI into possible misconduct by mostly high-ranking officers in Manhattan and Brooklyn. Commissioner William Bratton has disciplined eight officers so far in the larger FBI investigation.