In 2009, when Mueller ran the FBI, the bureau asked Russian oligarch Oleg Deripaska to spend millions of his own dollars funding an FBI-supervised operation to rescue a retired FBI agent, Robert Levinson, captured in Iran while working for the CIA in 2007.

http://thehill.com/opinion/white-house/387625-mueller-may-have-a-conflict-and-it-leads-directly-to-a-russian-oligarch