Robert Mueller had only been at the helm of the FBI for a week when terrorists successfully launched the biggest attack on U.S. soil since Pearl Harbor. Now, after 12 years of evolving threats against the United States, Mueller is preparing to say goodbye to the agency he has led for so long.
Even now, he loses sleep over what he described as the viable scenarios in which a terrorist could successfully take down a plane mid-flight, or biological weapons of mass destruction could fall into the hands of a terrorist or terrorist group.
However, he told "Power Players" that the recent threat that shuttered U.S. embassies overseas seems to have subsided.
"I think we have thwarted the initial attack plans, but we are monitoring the situation closely to determine whether there are still individuals out there that want to undertake much of the same or type of activity," Mueller told ABC News' Pierre Thomas in a rare interview.
Meanwhile, Mueller said FBI officials "are concerned" and "are monitoring" a flow of American fighters into Syria, where a near-civil war has reportedly killed more than 100,000.
"[When] you have individuals who are traveling to those venues you are concerned about, No. 1, about the associations they will make and, secondly the expertise they will develop and whether or not they will utilize those associations, utilize that expertise, to undertake an attack upon the homeland."
Mueller also recounted his reaction to the Boston Marathon bombing in April that killed three people and injured hundreds more.
"Needless to say, anyone who looked and saw those pictures was horrified at the killing, and the devastation and the injuries, the number of people who lost their limbs and legs and the like. You could not help but be horrified," Mueller said.
Mueller acknowledged that the older suspect, Tamerlan Tsarnaev, "had come to our attention before this occurred," but he insisted his agency did a thorough investigation and could not do anything more.
"In the future, we will have a case where the person has been on our radar screen, but we do not have the evidence and there is no outward evidentiary capability to bring the person into the criminal justice system," Mueller warned. "That is going to happen."
For more of the interview with Mueller, including how he says the government has improved in responding to mass shooting incidents, check out this episode of "Power Players."
ABC's Jack Cloherty and Alexandra Dukakis contributed to this episode.