U.S. President Donald Trump’s former campaign chairman Paul Manafort breached his plea deal by lying to federal investigators, U.S. prosecutors said in a court filing, signaling a potential setback to the special counsel’s probe into Russia’s meddling in the 2016 U.S. presidential election.

Manafort said in the same filing Monday that he disagreed with Special Counsel Robert Mueller’s assertion that he had lied, but both sides agreed the court should move ahead and sentence him for his crimes.

Without a pardon, the 69-year-old Manafort could spend the rest of his life in prison, experts said. The surprise development comes at a critical time for Mueller, who is expected to finalize a report in the coming months on the findings of his 18-month probe into Russia’s election meddling and possible collusion with the Trump campaign.

While not a fatal blow, the dissolution of Manafort’s plea agreement means Mueller is losing the contributions of a witness with deep ties to Russia and who ran the Trump campaign as it took off in mid-2016.

“It’s bad for the overall Mueller investigation,” said Patrick Cotter, a criminal defense lawyer in Chicago and former assistant U.S. attorney in New York. “He’s got one less witness today.”

Still, Cotter said, it did not mean Manafort’s cooperation had not been of any value, noting that he may have led Mueller to other sources of information.

(SD-Agencies)