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.

Richard Gates, former associate to Paul Manafort, leaves the Prettyman Federal Courthouse after a hearing February 23, 2018 in Washington, DC. Getty Images