Nightly News | August 22, 2013
>>> when he started the job as head of the fbi , nobody including robert mueller himself ever imagined what was about to happen just a week later. it changed america, the fbi , and the world. tonight, as he prepares to leave the job, the longest serving fbi director since j. edgar hoover speaks with our justice correspond enent pete williams .
>> reporter: robert mueller became fbi director a week before the 9/11 attacks. some question the sting cases, some people expressing only a desire to adak.
>> i ask, when do you know when that person who harbors an intent to harming citizens to picking up a pistol and killing 13 people.
>> i was looking at your picture in the lobby and you have more gray hair now than when you became director.
>> i'll also mention, so do you.
>> all right, fair enough.
>> reporter: he says it's been an intense 12 years and remembers the toughest times.
>> the hardest days that you have are those when you lose an agent in the course of duty.
>> reporter: and the moments of uncertainty, like the chaotic hours after the boston marathon bombing.
>> the investigation had more twists and turns in that very short period of time than in many investigations. you were up in the air in terms of whether you were doing the right thing, making the right decisions.
>> reporter: as for threats, mueller says threats remain high that terror groups might use an airliner. are you going to miss all this?
>> yes. great people, great challenge. it's been a great opportunity.
>> reporter: pete williams , nbc news at the fbi in washington.