Special counsel Robert Mueller has opened a federal investigation into the Democratic-leaning Podesta Group lobbying firm, and Tony Podesta, the brother of Hillary Clinton's former campaign manager, according to a report Monday morning.

The decision to focus on Podesta was born out of the probe into whether President Trump's campaign associate colluded with Russia, which is also looking into former Trump campaign chairman Paul Manafort's finances.

A Podesta Group spokesman told NBC the firm "is cooperating fully with the special counsel's office and has taken every possible step to provide documentation that confirms timely compliance. In all of our client engagements, the Podesta Group conducts due diligence and consults with appropriate legal experts to ensure compliance with disclosure regulations at all times — and we did so in this case."

Manafort and Podesta's firms worked together in a public relations campaign for the European Centre for a Modern Ukraine (ECMU) nonprofit, which is believed to be backed by the pro-Russian and oligarch-funded Ukrainian political group, Party of Regions.

The two are part of six firms who worked on the campaign that the special counsel is investigating.

Sources who spoke with NBC said the Podesta group became of significant interest because it may have violated the Foreign Agents Registration Act. FARA states Americans who lobby for foreign governments, leaders, or political parties must disclose their activities with the Justice Department.

Both Manafort and Podesta's firms only filed FARA disclosures following media reports of their associations with the ECMU.