Investigators probing whether law-enforcement officials were involved in an alleged multimillion-dollar prostitution ring on the Upper East Side are reviewing the activities of a sergeant formerly assigned to the precinct.

NBC New York has learned that the sergeant is not at this point suspected of wrongdoing, but Internal Affairs investigators are reviewing his memo book entries as a precaution because they have received information that he responded to calls at the brothel.

The prostitution ring was allegedly run by Anna Gristina, a suburban mother of four who has pleaded not guilty.

Prosecutors say the 44-year-old Gristina said she made millions of dollars from arranging trysts and claimed she had law-enforcement connections who'd let her know if charges loomed. Police have declined to comment.

But defense lawyer Peter Gleason said after court that Gristina was "a loving, caring mother" who was setting up a legitimate dating business.

Gristina is being held on $2 million bond.

A source familiar with the case tells NBC New York law-enforcement officials are trying to get the alleged madam to cooperate with them in order to bring charges against her clients.

If convicted, Gristina could face up to seven years in prison.