Tony Podesta, founder of the Podesta Group and brother to former Hillary Clinton campaign chairman John Podesta, will resign from his lobbying company in response to an investigation of the firm by special counsel Robert Mueller.

Podesta made the announcement Monday at a company-wide meeting, and informed clients that he would be stepping down.

It's not clear when Podesta's departure will be effective, according to Politico, which first reported the change.

It was reported last week that Mueller had opened a federal investigation into the Democratic-leaning Podesta Group lobbying firm and its founder.

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

Related: Experts say Manafort charges under foreign-agents law could spell trouble for Mike Flynn, Tony Podesta

A Podesta Group spokesman said 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.

Manafort and longtime business associate Rick Gates surrendered to the FBI Monday morning and were charged with 12 counts. The indictment lists the two as having made false and misleading Foreign Agents Registration Act statements, serving as an unregistered foreign principal, and seven counts of failure to file reports of foreign bank and financial accounts.