U.S. special counsel Robert Mueller has hit President Donald Trump’s former campaign manager with new conspiracy and other charges, including allegations that he set up a secret lobbying campaign involving former senior European politicians.

The new indictment against Paul Manafort was filed in U.S. District Court in Washington on February 23, just hours after Manafort's deputy, Rick Gates, pleaded guilty to related charges.

The flurry of filings followed an indictment a day earlier against the two men, and a related guilty plea by a London-based lawyer earlier in the week -- all indications that Mueller's investigation is quickening its pace and stepping up pressure to secure cooperation with the probe.

Mueller's investigation of Manafort and Gates has largely focused on the lobbying they did for a Russian-aligned Ukrainian political party, as well as various offshore companies and bank transfers that, prosecutors allege, were hidden from U.S. authorities.

'Former European Chancellor'

In the new document, Mueller charged that Manafort secretly enlisted "former senior European politicians" in 2012 and 2013 to lobby on behalf of the government of then-President Viktor Yanukovych, who was leader of Ukraine's Party of the Regions until he fled the country in February 2014.

The former politicians, who are not identified in the indictment, were paid more than 2 million euros for their work. They did not disclose they were being paid for lobbying, Mueller's office charged.

"The plan was for the former politicians, informally called the ‘Hapsburg group,’ to appear to be providing their independent assessments of government of Ukraine actions, when in fact they were paid lobbyists for Ukraine," the indictment stated.

The group operated from 2012 to 2013, and was managed by a "former European chancellor," who along with the other members lobbied U.S. legislators and White House officials, according to the indictment.

Although the indictment does not name any of the European politicians, the Associated Press previously reported that Manafort had worked with Mercury LLC, a U.S.-based lobbying firm that had employed former Austrian Chancellor Alfred Gusenbauer.

The members of the group were to "appear to be providing their independent assessments of government of Ukraine actions, when in fact they were paid lobbyists for Ukraine," the indictment said.

Gusenbauer, who served as chancellor of Austria in 2007-2008, and two other lobbyists involved in Manafort's campaign met with members of the U.S. Congress in 2013, according to Politico, which cites Justice Department disclosures retroactively filed last year by Mercury.

Manafort has repeatedly maintained his innocence.

Earlier on February 23, Gates appeared in federal court in Washington, where he entered a guilty plea to charges of conspiracy and lying to a federal law enforcement agent. Previously, he too had denied the charges.

Gates' change of heart indicated that he plans to cooperate with Mueller's team as they pursue the core of their mandate: investigating interactions between Trump associates and Russian officials, as well as alleged Russian meddling in the 2016 presidential election.

During his court hearing, prosecutors said they would recommend up to 71 months in prison for Gates, but that could be reduced based on his cooperation.

After Gates entered his plea, a Manafort spokesman issued a statement, repeating Manafort's position and criticizing Gates.

"I had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence," he said. "For reasons yet to surface he chose to do otherwise."

Gates is the fifth person to plead guilty in connection with Mueller's probe, including Trump’s first national security adviser, Michael Flynn.

Manafort served as Trump's campaign manager from March 2016 until August 2016, when he was fired after revelations about the extent of his work with the Party of the Regions.

Contradictory Explanations

After Manafort's firing, Gates continued working with the campaign, and assisted Trump's team in the transition after he won the election.

Mueller has also brought charges against three Russian companies and 13 Russians, including a St. Petersburg businessman with close ties to the Kremlin.

The question of Russian interference has shadowed Trump's administration since before he took office. He has repeatedly called for a more conciliatory approach toward Moscow, and has complained about Mueller's investigation, saying it has shown no evidence of collusion between his administration and Russia.

Flynn was fired by Trump in February 2017 after it emerged he had misled top White House officials about his interactions with Russia's ambassador to the United States.

Trump fired FBI director James Comey in May 2017, and has given contradictory explanations for it.

He said in an interview with NBC News in May 2017 that, at the time he decided to fire Comey, he was thinking that "this Russia thing with Trump and Russia is a made-up story. It's an excuse by the Democrats for having lost an election that they should’ve won."

After a congressional uproar, Mueller -- a former FBI director himself -- was appointed, overseen by the Justice Department's No. 2 official, Rod Rosenstein.

The U.S. intelligence community issued a report the same month that Trump took office that accused Russia of a widespread cyberhacking-and-propaganda campaign aimed at influencing the vote.

The indictment Mueller issued against more than a dozen Russian suspects this month asserted the campaign intended to support Trump's candidacy.