The special counsel investigating Russia’s role in the 2016 presidential race announced criminal charges Monday against three former campaign aides to Donald Trump, including his former campaign manager, marking an explosive new phase in the FBI investigation of the president’s inner circle.

One of the three repeatedly sought to arrange a meeting between then-candidate Trump and senior Russian officials in London or in Moscow, according to court documents. The meeting did not take place, but court documents describe an extensive effort by Russian officials to gain access to Trump’s operation.

Court papers disclosed that George Papadopoulos, a former foreign policy advisor to the Trump campaign, is cooperating with prosecutors led by special counsel Robert S. Mueller III, who is pursuing allegations that Trump’s aides cooperated with Russian officials seeking to influence the U.S. election.

Paul Manafort, who was Trump’s campaign manager, and Richard W. Gates III, who was Manafort’s top deputy and helped run Trump’s inauguration, were separately accused of a total of 12 counts of fraud, conspiracy and money laundering in a financial scheme that ran from 2006 to 2017.

In a court hearing Monday afternoon, Manafort and Gates both pleaded not guilty to the charges. Manafort was released on $10-million bail and Gates was released on $5-million bail. Both surrendered their passports and were ordered under house arrest.

While the case against Manafort and Gates carries the potential of long years of jail time, the case against Papadopoulos may be more significant for the White House. It is the first guilty plea related to dealings with Russia by someone connected with the Trump campaign.

The double-barrelled approach signaled that Mueller, a former FBI director, and his team of veteran organized-crime and white-collar prosecutors are willing to use a classic hardball approach — trying to pressure lower-level figures into cooperating and providing information — in their investigation of the Trump campaign.

“I think it definitely shows that Mueller is following a strategy of working on the perimeter and then moving to the center by finding people he can indict, and then seeking their cooperation,” said Jens David Ohlin, a Cornell Law School professor and expert in international criminal law. “There might be multiple rounds of this before you reach a smoking gun.”

Mueller was named in May as special counsel to investigate the Trump campaign’s possible dealings with Russian officials — as well as any other crimes they uncovered. On Monday, they showed that their reach would extend to prominent Democrats as well as Republicans.

Tony Podesta, a powerful Democratic lobbyist, announced he would step down from his firm, the Podesta Group, after it came under scrutiny in the Mueller investigation. The Podesta Group was one of two firms solicited by Manafort to work on a Ukraine lobbying campaign; Podesta’s brother, John Podesta, was Hillary Clinton’s campaign manager.

Court papers revealed that Papadopoulos had pleaded guilty in a closed-door court hearing on Oct. 5 to lying to the FBI about his contacts with people who claimed to have direct connections with Russian President Vladimir Putin and other senior Russian officials.

Papadopoulos admitted that he had met with an unnamed professor in London who told him that high-level Russian officials had damaging information on Clinton, including thousands of emails.

“They [the Russians] have dirt on her,” Papadopoulos said he was told during a meeting in London in April 2016, as Trump was gaining steam in the Republican presidential primaries.

After that, Papadopoulos, a 30-year-old think tank researcher who sought to become Trump’s contact with Moscow, communicated with someone who claimed to be Putin’s niece and another official who claimed to work at Russia’s foreign ministry, according to court documents. The woman, according to court documents, was not Putin’s niece.

Since his arrest at Dulles International Airport in July, Papadopoulos has met with Mueller’s team on “numerous occasions” to provide information as part of his plea deal, according to court filings.

The meeting in London is the second documented instance of someone with claimed connections to the Russian government offering damaging information about Clinton to the Trump campaign.

In June 2016, Manafort joined Donald Trump Jr. in a meeting in Trump Tower with a Russian lawyer and lobbyist, after Trump Jr. was offered damaging information about Clinton.

Manafort and Gates used offshore accounts and shell companies in Cyprus, the Seychelles and the Caribbean to hide $75 million, including payments for representing a pro-Kremlin political faction in Ukraine, to avoid paying U.S. taxes, according to the indictment.

“Manafort used his hidden overseas wealth to enjoy a lavish lifestyle in the United States, without paying taxes on that income,” the charges said.

The indictment says he laundered more than $18 million to pay for his expenses, including $5.4 million for renovations at his home on Long Island, N.Y.; $1.3 million for lighting and home entertainment at his home in Florida; $934,000 for antique rugs at his home in Virginia; and about $1.4 million at clothing stores in New York and Beverly Hills.

Manafort did not report the income to the government and denied to his tax preparer than he held any offshore accounts. The indictment also alleges that Manafort “defrauded” banks by making false statements to obtain cheaper mortgages, hiding the fact in one case that he was renting out a condominium in lower Manhattan via Airbnb.

Manafort also was charged with filing false reports to conceal the fact that he was acting as an unregistered foreign agent. The charges state that Manafort and Gates were agents for former Ukranian President Victor Yanukovich and his pro-Russian Party of Regions.

The indictment alleges that Gates moved $3 million from the offshore accounts to pay his mortgage, children’s tuition and interior decorating.

The indictment against Manafort and Gates doesn’t reference their work for the Trump campaign, a point Trump noted on Twitter.

“Sorry, but this is years ago, before Paul Manafort was part of the Trump campaign. But why aren’t Crooked Hillary & the Dems the focus?????” Trump wrote.

“… Also, there is NO COLLUSION!” he said in another tweet.

In a statement, Manafort’s lawyer, Kevin Downing, called the indictment “ridiculous” and said there was “no evidence” that the Trump campaign colluded with Russian government.

None of the charges released Monday make that claim.

Downing said Manafort was seeking to “further democracy” and help Ukraine “come closer” to the United States and the European Union. He said those activities ended in 2014, two years before Manafort joined the Trump campaign.

He said the Foreign Agent Registration Act has only been used six times to file criminal charges, and only one case resulted in a conviction. He also said the allegation that Manafort used offshore accounts “as a scheme to conceal” his money was “most ridiculous.”

Papadopoulos, a former researcher at the Hudson Institute, a conservative think tank, joined the Trump campaign in March after working for Ben Carson’s failed presidential bid. White House Press Secretary Sarah Huckabee Sanders described him Monday as a campaign “volunteer” whose role was “extremely limited.”

His lawyers declined to comment. “We will have the opportunity to comment on George’s involvement when called upon by the court at a later date. We look forward to telling all of the details of George’s story at that time,” said the statement by Thomas Breen and Robert Stanley of Chicago.

Los Angeles Times staff writers David S. Cloud, Evan Halper, Cathleen Decker, Noah Bierman and Brian Bennett and Katherine Skiba of the Chicago Tribune contributed to this report from Washington.

joseph.tanfani@latimes.com

Twitter: @jtanfani