The Turkish businessman at the center of an investigation into former national security adviser Michael Flynn was interviewed by prosecutors for Special Counsel Robert Mueller in recent months, but has refused to come to the U.S. to testify before a grand jury.

According to Hurriyet, one of Turkey’s largest newspapers, the businessman, Ekim Alptekin, the chairman of the Turkish-U.S. Business Council (TAIK), was interviewed by Mueller’s investigators during a trip to the U.S. in May. As head of TAIK, Alptekin hosted a conference at the Trump International Hotel from May 21-23.

Mueller, a former FBI director, was appointed as special counsel just days before the interview. He is currently running an expansive investigation into Russian interference in the presidential campaign, as well as the business activities of Flynn and former Trump campaign chairman Paul Manafort.

After meeting with Mueller’s investigators, Alptekin traveled back to Turkey. Mueller then subpoenaed him to appear before a federal grand jury in July, according to Hurriyet.

Alptekin’s alleged refusal to respond to the subpoena means that he is unable to visit the U.S. And because of that, he is reportedly planning to leave his position as head of TAIK, which is controlled by the Turkish government.

Mueller’s apparent interest in Alptekin stems from his business dealings last year with Flynn, a retired lieutenant general who operated the private consulting firm Flynn Intel Group while he was serving as an adviser to the Trump campaign.

Last August, Alptekin signed a three-month, $600,000 contract with Flynn Intel Group to investigate Fethullah Gulen, a political foe of Turkey’s president, Recep Tayyip Erdogan, who is living in exile in the U.S.

Flynn’s relationship with Alptekin was revealed by The Daily Caller several days after the election. On Election Day, Flynn wrote an op-ed in The Hill calling on the U.S. government to extradite Gulen. An investigation found that a Netherlands-based shell company registered to Alptekin had a lobbying relationship with Flynn Intel Group. (RELATED: Michael Flynn Is Lobbying For Turkish Businessman With Ties To Turkish Government)

Details of that relationship were not fully revealed until March, when Flynn registered with the Justice Department as a foreign agent of Turkey because of his work for Alptekin.

Mueller is reportedly interested in finding out whether Alptekin paid Flynn with money from the Turkish government. He is also reportedly investigating whether Flynn paid kickbacks to Alptekin during the course of their contract. (RELATED: Mueller Is Reportedly Investigating Whether Flynn Engaged In Kickback Scheme With Turkish Businessman)

Mueller’s investigators are suspicious of Alptekin’s claims that he paid Flynn with his own money, Hurriyet reported.

Flynn’s disclosures show that Alptekin paid Flynn Intel Group a total of $530,000. But Flynn Intel made two payments back to Alptekin totaling $80,000. Alptekin has claimed that the return payments were made for work that Flynn Intel did not do.

Alptekin did not return The Daily Caller’s request for comment.

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