Russia Probe: Mueller, New York Attorney General Collaborating on Trump Campaign Chairman Investigation

Robert Mueller pauses after making an opening statement at a Senate Judiciary Committee hearing on Capitol Hill in Washington, June 19, 2013. REUTERS/Larry Downing/File Photo

Special Counsel Robert Mueller has teamed up with New York Attorney General Eric Schneiderman in investigating Donald Trump's former presidential campaign chairman, Paul Manafort.

The cooperation is a sign the probe into the president's former campaign chairman is intensifying, several sources familiar with the issue told Politico.

Related: 'Are there any Russians here tonight?' Donald Trump mocks Robert Mueller probe at West Virginia rally

The Mueller and Schneiderman teams have spoken frequently in the last few weeks and shared evidence pertaining to the case. Both Shneiderman and Mueller have reportedly collected evidence relating to financial transactions, possible financial crimes and potential money laundering. However, it is unclear whether charges will be filed.

Sources close to Manafort have said his family and former business partners have been approached by members of the teams. A number of associates have received subpoenas relating to the investigation.

In late July, FBI agents searched one of Manafort's homes for documents as part of the investigation into possble Trump campaign collusion with Russia. His spokesman, Jason Maloni, said at the time of the raid that Manafort was cooperating with the investigators.

Schneiderman and President Trump have a fraught history. Last November, the New York prosecutor won a $25 million settlement following his investigation into fraud involving Trump University. Trump has attacked Schneiderman consistently on TV and social media.

Schneiderman's office also has been going through some of Trump's business transactions. The records could reportedly be shared with Mueller's team, although these inquiries are in their early stages.

The president has has regularly criticized investigations into connections between his campaign and Russian officials as a witch hunt. He has also apparently contemplated firing Mueller, despite the potentially toxic political fallout from such a move.

Speaking at a rally earlier this month, the president called the Russia probes "a fake story that is demeaning to all of us" and a "total fabrication." He made the comments just hours after it emerged a grand jury had been impaneled by Mueller and that subpoenas had been issued in connection with a June 2016 meeting involving Donald Trump Jr., a Russian lawyer and others.