Assistant District Attorney Eric Iverson revealed in Manhattan Criminal Court the panel had handed up an indictment against Louise Neathway, the New York Daily News reported.
Police have charged Neathway, 36, with extorting and cyberstalking Cashman, with whom she claims to have had a 10-month affair.
She pleaded not guilty to the charges and was being held on $200,000 bond, with her next court appearance scheduled for April 11.
Sources told the Daily News Cashman and Neathway both testified before the grand jury.
Prosecutors accuse her of extorting $6,000 from Cashman after threatening to reveal their alleged affair publicly, and of following that with a demand for another $15,000.
Cashman's wife of 16 years filed for divorce after news of Neathway's accusations surfaced.