The ex-campaign treasurer for U.S. Rep. George Santos will plead guilty on Thursday to an unspecified felony in connection with the sprawling federal investigation of financial irregularities surrounding the indicted New York Republican, prosecutors said.

Nancy Marks, a veteran Long Island political operative, served as the campaign treasurer and close aide to Santos during his two congressional bids. Marks resigned amid growing questions about Santos’ campaign finances and revelations Santos had fabricated much of his life story.