Special counsel Robert Mueller is focusing intensely on alleged interactions between former top Trump campaign official Rick Gates and political operative Roger Stone, one of President Donald Trump's closest confidants, according to sources with direct knowledge of the matter.
Stone, a longtime advisor to Trump, is apparently one of the top subjects of the Mueller investigation into potential collusion between the Kremlin and the Trump campaign, sources told CNBC on condition of anonymity.
The questions have been largely about what was discussed at meetings, including dinners, between Stone and Gates, before and during the campaign, said the sources, who have knowledge of the substance of the recent interviews.
In February, Gates pleaded guilty to two counts stemming from the Russia investigation, and he is cooperating with Mueller's probe.
The new developments indicate that Mueller's team is interested in Stone beyond his interactions with Wikileaks founder Julian Assange during the campaign.
An attorney for Stone, Robert Buschel, did not deny discussions took place between his client and Gates, but sought to downplay their importance.
"Roger Stone did not have any substantive or meaningful interaction with Rick Gates during or leading up to the 2016 campaign," Buschel told CNBC in a statement.
An attorney for Gates declined to comment. The special counsel's office declined to comment.
The link between Gates and Stone goes back to their work at what had been one of the most powerful lobbying firms in Washington, which was founded by Stone along with former Trump campaign chairman Paul Manafort. The special counsel's probe has yielded two indictments against Manafort, who is accused of several crimes, including bank fraud and conspiracy against the United States.
Gates joined the firm as an intern more three decades ago, and it is unclear how much work he did with Stone at the time.
The firm, called Black, Manafort, Stone and Kelly, was known for its work to help improve the image of controversial politicians, including Ferdinand Marcos of the Philippines, Mobutu Sese Seko of the Democratic Republic of the Congo and the Russian-aligned former president of Ukraine, Viktor Yanukovych.
Gates joined the Trump campaign in the spring of 2016 and became Manafort's deputy. It was there where he became close to many of Trump's confidants. He remained with the campaign even after Manafort's ouster. Gates then worked on Trump's inaugural committee and co-founded the pro-Trump nonprofit group America First Policies.
In March, Gates was pulled into the Mueller inquiry when the special counsel's office filed a motion that claimed the former campaign aide had contact with a former agent of the Russian intelligence service in 2016. This came after Gates pleaded guilty to lying and conspiring against the United States, which could lead to possibly six years in prison. A sentencing date has yet to be announced.
For Stone, this is another potential hurdle in an ongoing investigation that continues to focus on him, among others.
Sam Nunberg, a former Trump campaign advisor, also said he was asked about Stone's involvement with Wikileaks during his interview before Mueller's grand jury in March.
"Roger is certainly a subject," Nunberg said. "The fact that Roger hasn't been called in and the special counsel continues to ask questions about Roger's possible activities during the election shows that at the very least he's a subject."
Stone allegedly met with Assange, the Wikileaks founder, in August 2016. In an email leaked to The Wall Street Journal, Stone said, "I dined with my new pal Julian Assange last nite."
Stone has denied that he has met with the Wikileaks founder and said the email was in jest.
During the 2016 campaign, Wikileaks published emails allegedly stolen from the Democratic National Committee's servers by a Russia-linked hacker known as "Guccifer 2.0."