https://stonecoldtruth.com/muller-rosenstein-and-comey-the-three-amigos-from-the-deep-state/
He [Mueller] and Comey are good friends and former colleagues who worked hand-in-hand at the FBI and Department of Justice. Agents will tell you they were joined at the hip.
Comey regards his predecessor as a mentor, while Mueller considers Comey his protégé. When Comey was appointed to succeed Mueller as FBI Director, both men appeared together and were effusive in their praise of one another. Their relationship is not merely a casual one. It is precisely the kind of association which ethical rules are designed to guard against.