Special counsel Robert Mueller is investigating whether former White House National Security Adviser Mike Flynn plotted to kidnap Turkish cleric Fethullah Gülen, who lives in Pennsylvania, and forcibly return him to Turkey for $15 million. The Turkish government views Gülen as a political enemy and has accused him of plotting a failed military coup in 2016.

The investigation centers on a December meeting between Flynn and representatives of the Turkish government, including Turkish President Recep Tayyip Erdogan’s son-in-law and Turkey’s foreign minister. Flynn was working as a “foreign agent” for the Turkish government last fall, even as he served as a top adviser to Donald Trump’s campaign—and failed to disclose his lobbying efforts as required by law.

Both Flynn and his son, Mike Flynn, are under investigation.