WASHINGTON — Rick Gates, who helped manage Donald Trump’s presidential campaign after making millions of dollars advising Ukraine’s pro-Kremlin government, pleaded guilty Friday to conspiracy against the United States and lying to federal agents, becoming the latest former Trump aide to cooperate with prosecutors in the sprawling Russia investigation.

Court papers reveal Gates lied to special counsel Robert Mueller and the FBI as recently as Feb. 1, when he already was negotiating with prosecutors about the raft of criminal charges he was facing.

Gates’ original three lawyers applied to withdraw from the case the same day, a hint of a behind-the-scenes drama that preceded Friday’s denouement.

Gates, who appeared in court before U.S. District Judge Amy Berman Jackson, is the fifth person to plead guilty to charges stemming from the Mueller probe, and the first to confess to more than one criminal charge.

While Gates, 45, could face more than five years in prison on the two charges, he is expected to be sentenced to about 18 months, according to a person familiar with the plea negotiations.

Mueller has proven adept at securing cooperation from Trump’s former associates as he investigates Russian meddling in the 2016 election, and whether anyone in the Trump campaign assisted in that operation or committed other potential crimes, including obstruction of justice while in the White House.

Gates’ plea poses the biggest immediate risk to Paul Manafort, who was Trump’s campaign chairman and Gates’ longtime business partner.

Prosecutors had filed several dozen charges against Manafort and Gates, including tax evasion, bank fraud and conspiracy, mostly stemming from their alleged attempts to hide more than $30 million from their consulting and lobbying work for the Kremlin-backed government in Ukraine.

They both pleaded not guilty when the first indictment was unveiled Oct. 30. Gates now is expected to testify against Manafort, who said Friday that he is innocent of the charges.

“Notwithstanding that Rick Gates pled today, I continue to maintain my innocence,” Manafort said in a statement. “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. This does not alter my commitment to defend myself against the untrue piled up charges contained in the indictments against me.”

Several hours later, the special counsel’s office unveiled a superseding indictment against Manafort that also accused him of secretly paying former European politicians to advocate for Ukraine, part of an undisclosed effort to lobby members of Congress and the Obama administration.

None of the charges against Gates and Manafort cite Russian meddling in the presidential election, which was the impetus for Mueller’s investigation. But the allegations extended through 2017 and thus overlapped with the pair’s work in the top ranks of the Trump campaign, according to court documents.

It’s unknown whether Gates has more to offer beyond testimony against Manafort. He served as a senior adviser to the Trump campaign after Manafort stepped down as campaign chairman in August 2016, helped organize Trump’s inauguration in January 2017, and had access to the White House as an outside adviser in the early weeks of the administration.

A person familiar with the negotiated plea said Gates can expect “a substantial reduction in his sentence” from his cooperation with the special counsel’s office. If Gates were convicted of all the earlier charges, he could have faced years in prison.

Gates is scheduled to return to court May 14 for a status hearing.

Gates’ guilty plea for lying to investigators stems from his account of a dinner discussion on March 19, 2013, between Manafort and Rep. Dana Rohrabacher, R-Calif., who has been a strong advocate for better U.S. relations with Russia.

Three weeks ago, as he was negotiating his plea deal, Gates told the FBI that Manafort and an unnamed lobbyist had told him they didn’t talk to Rohrabacher about Ukraine.

But, according to a court filing Friday, Gates had helped Manafort prepare a report that “memorialized for Ukraine leadership the pertinent Ukraine discussions that Manafort represented had taken place at the meeting.”

Rohrabacher declined to comment when contacted by a reporter Friday.

Mueller was appointed last May and has already scored several successes.

Former White House national security adviser Michael Flynn pleaded guilty Dec. 1 to lying to investigators about his communications with a Russian diplomat during the presidential transition. He admitted to discussing sanctions that former President Barack Obama had implemented to punish Moscow for meddling in the campaign.

George Papadopoulos, a former campaign foreign policy adviser, also pleaded guilty to lying last year. He had falsely denied contacts with Russians who claimed to have “thousands of emails” on Hillary Clinton,

Alex van der Zwaan, a former lawyer at a prominent U.S. law firm who worked on a report on behalf of Ukraine’s pro-Russia government, pleaded guilty Tuesday. He lied to investigators about his communications with Gates.

A separate indictment from Mueller’s team last week charged 13 Russians and three Russian companies with illegally using social media, stolen IDs and other tactics in an effort to sway U.S. voters to support Trump and vote against Hillary Clinton.

Richard Pinedo, a California resident, pleaded guilty to identity theft as part of that investigation.

Correspondent Sarah D. Wire contributed.

chris.megerian@latimes.com.