are buried. robert mueller opens up to abc news, including what he sees as the greatest threat we're facing. >> reporter: outgoing fbi director robert mueller is offering a sober assessment. he told pierre thomas what keeps him up at night. >> the possibility of a plane being taken down. a weapon of mass destruction in the hands of a terrorist. >> reporter: mueller warned about another danger, americans joining islam mick radicals. do you believe some americans have made their way to syria? yes. >> reporter: he says fror groups and individuals have -- terror groups and individuals have my great to many areas in the middle east. >> it changed from september 11th where it was al qaeda, afghanistan, pakistan. it has gone to yemen, the arabian peninsula. >> reporter: mueller said law enforcement must do a better job identifying people before they act. >> i do believe we're better at responding, but when it goes to preventing at the outset, it's a difficult position on how you prevent that...