A mystery solved: Secret Mueller case involves Russia-dealing law firm that worked for oligarch tied to Putin
Official FBI photo of Robert S. Mueller.

A law firm that's been hired by a Russian oligarch and other Russian interests is involved in a challenge to special counsel Robert Mueller's mysterious grand jury subpoena.

CNN reported Wednesday that Alston & Bird, an international law firm with offices in the United States, Europe and Asia, is involved in a foreign government's challenge to Mueller's secret grand jury subpoena.

The revelation of the firm's identity "is among the first details revealed about a case that's progressed to the Supreme Court under extreme secrecy," CNN reported.

Alston & Bird, the report added, began representing Russian oligarch Oleg Deripaska in 2003 to help him reinstate his visa and was on his payroll in the following years -- the same time the oligarch gave Paul Manafort a $10 million loan.

DC white collar attorney Ted Kang and North Carolina appellate attorney Brian Boone are also involved in the case, the report noted, though it's unclear if they represent the company or another party.

On his website, Kang claims to represent multiple "entities and individuals" associated with Mueller's investigation and Boone has in the past represented the Republican National Committee, CNN noted.