The ex-campaign treasurer for U.S. Rep. George Santos is scheduled to enter a guilty plea to an unspecified felony in connection with the sprawling federal investigation of financial irregularities surrounding the indicted New York Republican, prosecutors say.

Nancy Marks is a veteran Long Island political operative. Marks 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.

Marks’ plea is scheduled to take place in a Central Islip courtroom on Thursday afternoon. It comes as Santos faces a 13-count federal indictment centered on charges of money laundering and lying to Congress in an earlier financial disclosure.

The investigation of the first-term congressman has also engulfed Marks, a key behind-the-scenes figure in Long Island Republican politics who built a business as a treasurer and consultant to dozens of local, state and federal candidates.

Marks has faced questions about the congressman’s unusual campaign filings, including a series of $199.99 expenses, just below the legal limit for disclosure. Santos, in turn, has sought to pin the blame for his unexplained finances on Marks, who he claims “went rogue” without his knowledge.

Any deal with prosecutors that requires Marks to testify in the case against Santos could be a severe blow to the Republican, who faces charges that he embezzled money from his campaign, lied in financial disclosures submitted to Congress and received unemployment funds when he wasn’t eligible.

While Santos has admitted fabricating key parts about his purported background as a wealthy, well-educated businessman, questions remain about what he did for work, as well as the true source of more than $700,000 he initially claimed to have loaned his campaign from his own personal fortune.

Santos has pleaded not guilty to charges he duped donors, stole from his campaign and lied to Congress about being a millionaire, all while cheating to collect unemployment benefits he didn’t deserve. He has defied calls to resign.

A formal complaint filed by the Campaign Legal Center with the Federal Election Committee alleges that unknown groups may have illegally funneled money into the Santos campaign. The complaint, filed last January, named Marks along with Santos.