Special counsel Robert Mueller issued a subpoena compelling a business associate of ousted national security adviser Michael Flynn’s Turkish lobbying client to testify before a grand jury earlier this month, ProPublica reported Friday.

Sezgin Baran Korkmaz, a Turkish equity investor, was ordered to appear on Sept. 22 before the grand jury in Washington, D.C., according to the report.

“The grand jury is conducting an investigation of possible violations of federal criminal laws involving the Foreign Agents Registration Act, among other offenses,” read a letter accompanying the subpoena, which was obtained by ProPublica.

Korkmaz is reportedly a close ally of Ekim Alptekin, the Turkish businessman who entered into a $600,000 contract with Flynn Intel Group, Flynn’s now-defunct lobbying firm, to lobby for Turkish interests late in the 2016 campaign. The firm belatedly registered with the Justice Department under the Foreign Agents Registration Act (FARA) for that work, conceding that it “could be construed to have principally benefited the Republic of Turkey.”

Korkmaz did not return ProPublica’s requests for comment. But an unnamed source close to the investigation told the publication that Korkmaz’s relationship with Alptekin could be the reason he was asked to testify, as Mueller’s team wants to suss out the original source of the money behind the lobbying effort.

That campaign involved researching and producing negative PR materials about exiled Turkish cleric Fetullah Gulen, who Turkish President Recep Tayyip Erdogan claims coordinated a failed coup against him in summer 2016. Flynn’s failure to register under FARA for that work is one of several areas of interest for Mueller’s investigators. They’re also looking into Flynn’s failure to disclose both meetings with Russian officials and funds he received from Russia-linked firms.

Korkmaz apparently has his own business interests in Russia. His company, SBK Holding, has “major investments” in the Russia energy sector, according to ProPublica. The publication also noted that some of Korkmaz’s previous colleagues and investing partners have been investigated for criminal activity by U.S. authorities.