Tony Podesta and the Podesta Group are now the subjects of a federal investigation being led by Special Counsel Robert Mueller, three sources with knowledge of the matter told NBC News.

The probe of Podesta and his Democratic-leaning lobbying firm grew out of Mueller's inquiry into the finances of former Trump campaign chairman Paul Manafort, according to the sources. As special counsel, Mueller has been tasked with investigating possible collusion between the Trump campaign and Russia.

Tony Podesta is the chairman of the Podesta Group and the brother of John Podesta, Hillary Clinton's presidential campaign chairman. John Podesta is not currently affiliated with the Podesta Group and is not part of Mueller's investigation.

In a statement, a spokesman for the Podesta Group said the firm always "conducts due diligence ... to ensure compliance" and "is cooperating fully with the Special Counsel's office and has taken every possible step to provide documentation that confirms timely compliance."