Several billionaires with deep ties to Russia attended exclusive, invitation-only receptions during Donald Trump’s inauguration festivities, guest lists obtained by ABC News show.

These powerful businessmen, who amassed their fortunes following the collapse of the Soviet Union -- including one who has since been sanctioned by the U.S. Treasury Department -- were ushered into events typically reserved for top donors and close political allies and were given unprecedented access to Trump’s inner circle.

Their presence has attracted the interest of federal investigators probing Russian efforts to influence the 2016 presidential election, three sources with knowledge of the matter said.

Matthew Olsen, a former senior national security official who now serves as an ABC News consultant, said their presence at inaugural events is “very concerning.”

“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.”

The presence of people with Kremlin ties in Washington for Trump’s inaugural celebration was first reported by The Washington Post. But the guest lists obtained by ABC News offer a new glimpse at the level of access granted to several well-connected oligarchs.

Several donated enough to the Presidential Inaugural Committee to qualify for tickets to a “Candlelight Dinner” in Washington’s Union Station on the eve of the inauguration, a perk for $1 million contributors, the list of attendees show. Guests were treated to a preview performance by singer Jackie Evancho, a one-time runner-up on "America's Got Talent," who would go on to sing at the inauguration the following day.

A handful celebrated Trump’s surprise victory at the black-tie “Chairman’s Global Dinner,” an exclusive 500-guest affair held the same night in the Greek-columned Andrew W. Mellon Auditorium whose attendees included close Trump friends, high-ranking campaign aides, and an array of foreign ambassadors and dignitaries. The inaugural committee treated them to a dinner of butter-poached lobster, coffee-crusted beef tenderloin and chocolate raspberry dome. For entertainment, casino mogul and Trump pal Steve Wynn flew in Las Vegas show girls to perform a Broadway-style rendition of Frank Sinatra’s “New York, New York.”

According to a source with knowledge of the congressional investigations, at least one oligarch was ushered into Statuary Hall in the U.S. Capitol for the traditional Inaugural Day luncheon, hosted by the Joint Congressional Committee on Inaugural Ceremonies – an event typically out of reach to donors and even most rank-and-file members of Congress.

“It’s incredibly unusual,” said Stephen Kerrigan, who planned the 2009 and 2013 Obama inaugural festivities and said every guest was scrutinized for foreign ties. “Particularly if they were going to be within arm’s reach of the President, they went through an intensive vetting process.”

Names on the guest lists of the Candlelight Dinner included Victor Vekselberg, the billionaire head of the global conglomerate Renova Group, who was later sanctioned by the U.S. Treasury “for operating in the energy sector of the Russian Federation economy,” and his American cousin Andrew Intrater. Late last year, Vekselberg was stopped at a New York airport by federal agents working for Special Counsel Robert Mueller and questioned, according to the New York Times. The Times also reported that Intrater was separately interviewed by the special counsel.

Vekselberg and Intrater were seated at the dinner next to Michael Cohen and his family, according to a source familiar with the arrangements. Intrater’s firm, Columbus Nova, later signed Cohen to a $1 million consulting contract. Emails to Renova Group went unanswered. A spokesperson for Intrater declined to comment when reached by ABC News.

Investigators for both the Mueller team and the congressional probes have also expressed interest in several of the other Russian guests who had no obvious place in Trump’s diplomatic orbit, three sources told ABC News.

Some had given money to past Republican candidates, including Leonard Blavatnik, a Ukrainian-born billionaire philanthropist who holds American and British citizenship. He appeared on guest lists for both the Candlelight Dinner and the Chairman’s events. His global company, Access Industries, which holds a majority stake in Amedia, Russia’s largest producer of television series, gave $1 million to the inaugural fund.

A spokeswoman at Access Industries directed questions from ABC News to a company official who was not immediately available for comment.

Alexander Shustorovich, a Russian-born American music industry mogul with a background in nuclear energy, also donated $1 million to the Trump inauguration and attended Chairman’s Dinner. Nearly two decades ago, the Republican National Committee returned a six-figure political contribution from Shustorovich because of his past ties to the Russian government, according to the Wall Street Journal, which quoted a GOP official saying it wasn’t “worth the trouble.”

In response to questions from ABC News, John Evans, the chief operating officer of IMG Artists, a company owned by Shustorovich, said the wealthy businessman was maligned by that report and has since made sizable gifts to the Republican Party. He said Shustorovich has actually been under attack in Russia, where he has faced allegations of being an agent of American influence.

“Alex Shustorovich has not been contacted by anyone regarding his attendance at the Trump inauguration, except the press,” Evans said.

A Soviet-born billionaire Alexander Mashkevitch, who built a metals and mining empire in Kazakhstan, appears on the guest list for the Candlelight Dinner. Flight records show his private jet flew into Washington, D.C. on Jan. 19, 2017 and out two days later. European media reports indicate he settled money laundering charges with Belgian authorities in 2011 with no finding of wrongdoing by him. He has held an ownership stake in Eurasian Natural Resources Corporation, one of the world’s largest mining and metals concerns, based in London.

ABC News has attempted to reach Mashkevitch through his companies for comment but they have not responded.

Russian ambassador Sergei Kislyak joined numerous other ambassadors on the invite list for the Chairman’s dinner.

Guests dined alongside some of Trump’s closest friends and advisors, including private equity investor Thomas Barrack Jr., who chaired the inaugural fundraising effort; investment tycoon Carl Ichan, a longtime Trump friend; and Wynn, the casino boss who also became the Republican National Committee’s finance chairman. He stepped down earlier this year amid a swarm of sexual misconduct allegations, which he has denied.

Trump advisors who have faced criminal charges – such as former National Security Adviser Michael Flynn, who pled guilty to lying to the FBI, and former campaign manager Paul Manafort, who awaits trial on charges related to financial crimes, were at the dinners. Manafort pleaded not guilty and is in jail awaiting trial.

Invited guests also included since-departed White House aide Steve Bannon and campaign advisor and Cambridge Analytica CEO Alexander Nix, both of whom have since been questioned in the Special Counsel’s probe.

The Trump inauguration brought in more than $107 million, double the amount of President Obama, to finance a week of festivities that was filled with far fewer events than past inaugurations – only three Presidential balls. In May, ABC News reported that the Special Counsel had questioned several witnesses about millions of dollars in donations from donors with connections to Russia, Saudi Arabia, the United Arab Emirates and Qatar.

Editor’s note: An earlier version of this story incorrectly stated that Andrew Intrater was stopped at a New York airport alongside Victor Vekselberg. Only Vekselberg was stopped. The person who declined to comment on Intrater’s behalf was his spokesperson, not his lawyer.