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.
Manafort had organized a public relations campaign for a non-profit called the European Centre for a Modern Ukraine (ECMU). Podesta's company was one of many firms that worked on the campaign, which promoted Ukraine's image in the West.
The sources said the investigation into Podesta and his company began as more of a fact-finding mission about the ECMU and Manafort's role in the campaign, but has now morphed into a criminal inquiry into whether the firm violated the Foreign Agents Registration Act, known as FARA.