Former Special Counsel Robert Mueller condemned President Donald Trump‘s decision to commute the sentence of his longtime ally and advisor Roger Stone, saying that “his conviction stands.”

“A jury later determined he lied repeatedly to members of Congress. He lied about the identity of his intermediary to WikiLeaks. He lied about the existence of written communications with his intermediary. He lied by denying he had communicated with the Trump campaign about the timing of WikiLeaks’ releases. He in fact updated senior campaign officials repeatedly about WikiLeaks. And he tampered with a witness, imploring him to stonewall Congress,” Mueller wrote in a Washington Post op-ed published Saturday evening.

The former special counsel, who led the investigation into potential collusion between Trump’s 2016 campaign and Russia, continued: “The jury ultimately convicted Stone of obstruction of a congressional investigation, five counts of making false statements to Congress and tampering with a witness. Because his sentence has been commuted, he will not go to prison. But his conviction stands.”

Last November, Stone was convicted on seven charges including lying to Congress and witness tampering as part of Mueller’s investigation. Trump had voiced his support for Stone for several months on Twitter, including accusing the jury forewoman and judge assigned to the case of being politically biased. He commuted the sentence days before Stone was set to report to federal prison and begin his 40-month sentence.

Mueller’s final report found that Russia had interfered in the election, but did not find any evidence of collusion between the foreign country and the Trump campaign.

“Russian efforts to interfere in our political system, and the essential question of whether those efforts involved the Trump campaign, required investigation,” Mueller wrote. “In that investigation, it was critical for us (and, before us, the FBI) to obtain full and accurate information. Likewise, it was critical for Congress to obtain accurate information from its witnesses. When a subject lies to investigators, it strikes at the core of the government’s efforts to find the truth and hold wrongdoers accountable. It may ultimately impede those efforts.”

Mueller, who had largely remained silent since the release of his report, underscored that the investigation was not politically motivated.

“We made every decision in Stone’s case, as in all our cases, based solely on the facts and the law and in accordance with the rule of law. The women and men who conducted these investigations and prosecutions acted with the highest integrity. Claims to the contrary are false,” Mueller wrote.