To a stark warning of the outgoing director of the fbi. After running the bureau for 12 years, robert mueller is stepping down and speaking out about threats that keep him up at night. Abc's pierre... See More
To a stark warning of the outgoing director of the fbi. After running the bureau for 12 years, robert mueller is stepping down and speaking out about threats that keep him up at night. Abc's pierre thomas sat down with him and has the story. Reporter: As the bloodshed continues in syria -- Reporter: The fbi director sees a potential threat here at home. He says americans are now joining islamic radicals fighting in syria. Possibly including al qaeda. Is it your sense that some americans have made their way to syria? Yes. Reporter: It must be concerning. Yes. You are concerned 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 attack the homeland. Reporter: In an interview at the fbi's strategic command center, during his final days in office, mueller made clear that the threat of terror at home and abroad, remains urgent. What in the final days worries you most? What keeps you up at night now? The possibility of a plane being taken down. And the possibility of a weapon of mass destruction in the hands of a terrorist or a terrorist group who are not afraid to launch it. Chemical, biological and cyber. The ability to impact substantially the financial markets or the energy sector. Reporter: Mueller suggested that the boston marathon bombing was a painful reminder that the fbi's work is never done. The killing and the devastation, and the injuries, the number of persons that lost limbs and legs and the like. You could not help but be horrified. Reporter: The director said the boston attack was not directed from overseas but was homegrown, which he says is a major ongoing threat. Something he fears cannot always be stopped. Lara? Pierre, thank you. It's scary to hear the director's fears. Makes it all that more real.
This transcript has been automatically generated and may not be 100% accurate.