The CIA is my guess. Actually the whole national security administration. They have a vested interest in Hillary Clinton. She is a conniving war monger and conniving is what the CIA does.
Under Clinton the CIA was integral in overthrowing Gaddafi and Mubarak. And the creation of ISIS to overthrow Bashar. Even though HRC was not in the State Department when the CIA was integral in overthrowing Yanukovych of Ukraine, it was right out of her, and their playbook.
And Comey knows if anyone can get away with assassinating him or maybe they threatened his family, it is the CIA.