Former FBI Director Robert Mueller is expanding the investigation into ties between Trump campaign officials and Russia to include a grand jury investigation into former national security adviser Mike Flynn's work in Turkey, according to a report.

Federal prosecutors are investigating a deal between Flynn and Ekim Alptekin, a Turkish businessman who runs the Netherlands-based Inovo BV, as part of the grand jury probe, according to Reuters.

Flynn received $530,000 from Inovo between September 2016 and November 2016 and was tasked with producing a documentary and research on Fethullah Gulen, an exiled Turkish cleric who now lives in the United States. Turkish President Recep Tayyip Erdogan believes Gulen was the mastermind behind last year's failed coup against him.

Flynn never disclosed the work he did for Alptekin, but he retroactively filed a lobbying disclosure form in March.

In a letter accompanying the form, Flynn's lawyer said his work "could be construed to have principally benefited the republic of Turkey."

A grand jury in Virginia has now issued subpoenas to Flynn's business associates seeking bank records, documents and communications related to Flynn, his company, Alptekin, and Inovo.

Trump's team first said it was worried about Flynn's work with Inovo on Nov. 18, the day after Trump appointed Flynn to serve as his national security adviser. The president fired Flynn in February.

In addition to the FBI's probe, Flynn is also being investigated by the House and Senate Intelligence Committees, both of which subpoenaed records from Flynn.

The retired general invoked his Fifth Amendment right last week, but indicated this week he would hand over relevant documents to the Senate panel.