Tony Podesta is seen in this undated photo.  (Facebook)

Tony Podesta, the older brother of Hillary Clinton campaign chairman John Podesta and co-founder of the onetime lobbying powerhouse the Podesta Group, was thrust back into the political spotlight after President Donald Trump pointedly questioned why he had not been "charged and arrested."

Trump focused on Podesta in one of a series of tweets Sunday attacking Special Counsel Robert Mueller's investigation into alleged collusion between the Trump campaign and Russian officials.

In October, reports emerged that Mueller was investigating the Podesta Group over its lobbying work on behalf of a nonprofit group called the European Centre for a Modern Ukraine (ECMU). According to the special counsel's indictment of former Trump campaign manager Paul Manafort and associate Rick Gates, the Brussels-based ECMU functioned as a "mouthpiece" for Viktor Yanukovych, Ukraine's pro-Russian president between 2010 and 2014.

The indictment went on to allege that Manafort and Gates enlisted the Podesta Group and one other firm to lobby on behalf of the ECMU -- the "nominal client," in the words of the indictment -- so that Manafort and Gates could continue their work on Yanukovych's behalf without making the required legal disclosures to the Justice Department.

The indictment alleged that the Podesta Group was not paid directly by the ECMU, but rather through offshore bank accounts controlled by Manafort and Gates. It also claimed that Gates gave the Podesta Group false talking points in order to cover his and Manafort's tracks about their work for Yanukovych and ECMU.