President Trump’s former campaign chairman Paul Manafort pleaded guilty Friday to two criminal charges under terms of a plea deal that includes his cooperation as a potential witness for special counsel Robert S. Mueller III.

The decision by Manafort to provide evidence in exchange for leniency on sentencing is a stunning development in the long-running probe into whether any Trump associates may have conspired with Russia to influence the 2016 election.

Manafort’s defenders have long insisted that he would not cooperate with Mueller, and didn’t know any incriminating information against the president.

Prosecutor Andrew Weissmann said at the beginning of Friday’s plea hearing that Manafort had agreed to cooperate with investigators.