WASHINGTON — A former senior adviser to President Donald Trump’s election campaign pleaded guilty Friday to federal conspiracy and false-statements charges, switching from defendant to cooperating witness in the special counsel’s probe of Trump’s campaign and Russia’s election interference.

The plea by Rick Gates revealed that he will help special counsel Robert Mueller’s investigation in “any and all matters” as prosecutors continue to probe the 2016 campaign, Russian meddling and Gates’ longtime business associate, one-time Trump campaign chairman Paul Manafort.

With his cooperation, Gates gives Mueller a witness willing to provide information on Manafort about his finances and political consulting work in Ukraine, and also someone who had access at the highest levels of Trump’s 2016 presidential campaign.

Gates, 45, of Richmond, Va., made the plea at the federal courthouse in Washington. He admitted to charges accusing him of conspiring against the U.S. government related to fraud and unregistered foreign lobbying as well as lying to federal authorities in a recent interview.

The plea came a day after a federal grand jury in Virginia returned a 32-count indictment against Gates and Manafort, Trump’s former campaign chairman, accusing them of tax evasion and bank fraud. Gates is the fifth defendant to plead guilty in Mueller’s investigation.

The indictment in Virginia was the second round of charges against Gates and Manafort, who were initially charged last October with unregistered lobbying and conspiring to launder millions of dollars they earned while working on behalf of a pro-Russian Ukrainian political party.

Manafort continues to maintain his innocence.

“I had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence. For reasons yet to surface he chose to do otherwise,” Manafort said Friday. “This does not alter my commitment to defend myself against the untrue piled-up charges contained in the indictments against me.”

In court filings, Gates gradually began to show the strain the case was placing on him and his family. He frequently pleaded with U.S. District Court Judge Amy Berman Jackson for leniency in his house arrest to let him attend sporting events with his four children. Even on Friday, ahead of his plea, Gates had asked the judge to let him take his children to Boston for spring break so they could “learn about American history in general, and the Revolutionary War in particular.”

Under the terms of the plea, Gates is estimated to face between 57 and 71 months behind bars. Prosecutors may seek a shortened sentence depending on his cooperation.

Gates’ decision marks the fifth publicly known guilty plea in the special counsel probe into possible collusion between the Trump campaign and the Kremlin during the 2016 campaign. The charges to which Gates is pleading guilty don’t involve any conduct connected to the Trump campaign.