Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s office is investigating how several billionaires tied to Russia were given access to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s exclusive, invitation-only inauguration parties, ABC News reported Thursday.

The outlet obtained guest lists to festivities, many including businessmen who got wealthy after the fall of the Soviet Union. Three sources told ABC News the lists are now of interest to federal prosecutors.

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One Trump guest, oligarch Viktor Vekselberg, who has close ties to Russian President Vladimir Putin, has even been sanctioned by the Treasury Department in response to what U.S. officials called Moscow’s broad destabilizing activities during the presidential race.

The men were taken to exclusive events that were reserved for members of Trump’s inner circle and top donors.

Matthew Olsen, a former national security official turned ABC News consultant, said appearing at the new president's inauguration is a Russian strategy.

“This reflects a Russian strategy of gaining access to our political leaders at a time when they are just forming a government,” Olsen said. “They don’t need to be spies in the James Bond sense. They are powerful people with significant wealth who are in a position to exert influence on U.S. policy makers. And they’re in a position to report back to Russian intelligence services on what they’re able to learn.”

One oligarch was reportedly even inside the Capitol for the Inauguration Day luncheon with the Joint Congressional Committee on Inaugural Ceremonies, according to a source familiar with the congressional investigations.

ABC noted that that event is so exclusive that it is sometimes difficult for even members of Congress to get an invitation.

Several Russian oligarchs received tickets to a “Candlelight Dinner” in D.C.’s Union Station the night before the inauguration — open only to those who contributed at least $1 million to the inauguration.

Vekselberg sat at a table next to Trump’s former personal lawyer Michael Cohen and his family, a source told ABC News.

Vekselberg reportedly got an invite to the party through his cousin Andrew Intrater, an American businessman who also donated to Trump’s inauguration.

Both Vekselberg and Intrater were questioned by Mueller’s agents at a New York airport last year.

Trump’s inaugural committee announced in April 2017 that it had raised $106.7 million to pay for the festivities.

Mueller is currently investigating Russian meddling in the 2016 presidential election, as well as any possible coordination between Moscow and members of Trump's campaign.