Ousted national security adviser Michael Flynn’s well-compensated lobbying for a Turkish businessman during the 2016 campaign is now part of special counsel Robert Mueller’s investigation into Russia’s election interference, Reuters reported Friday.

Three sources told Reuters that Mueller was assuming control over an ongoing federal grand jury investigation into Flynn’s lobbying work, which forced him to retroactively register as a foreign agent after leaving the White House.

Flynn’s consulting firm, Flynn Intel Group, was paid $530,000 by Inovo BV, a Dutch firm owned by Turkish businessman Ekim Alptekin, to research and produce negative PR materials about exiled Turkish cleric Fetullah Gulen. Turkish President Recep Tayyip Erdogan believes Gulen orchestrated a failed coup against him last summer from his compound in Pennsylvania.

Reuters reported that the grand jury investigation, run out of the Eastern District Court of Virginia, has issued subpoenas for Flynn and his business associates. The subpoenas requested bank records, documents and communications related to Flynn, Flynn Intel Group, Alptekin and Inovo, per the report.

Mueller’s investigation, which includes determining whether there was any collusion between Trump campaign associates and Kremlin operatives, is also looking into Flynn’s conversations with Russian officials. Flynn had multiple conversations with the Kremlin’s ambassador to the U.S. about lifting the Obama administration’s economic sanctions against Russia. Former acting attorney general Sally Yates testified that the Justice Department believed Flynn was “compromised with respect to the Russians” because he knowingly misrepresented those conversations to other Trump administration officials.