The out-going head of the FBI fears another 9/11-style attack on the US, saying the fallout from the Arab Spring has bred a number of "violent extremists".
Robert Mueller, who leaves his post on September 4th, said that he fears terrorists will target planes, or attack America using a weapon of mass destruction. He warned that the US does not have the capability to defend a cyberattack on the country's energy sector.
Mr. Mueller said "I always say my biggest worry is ... an attack on a plane. And secondly, it's a weapon of mass destruction in the hands of a terrorist and that includes a cyber capability that trumps the defences that we have."
The FBI director said terrorism has shifted from Osama bin Laden's global brand in the aftermath of the 2001 attacks to splintering threats arising in the fallout from Middle East uprisings.
He said "Every one of these countries now has cadres of individuals who you would put in the category of extremists, violent extremists, and that will present threats down the road."
Mr. Mueller, who started as FBI head a week before 9/11, has been credited as the architect of the bureau's transformation into a terrorism-fighting agency in his 12 years at the helm.
Mueller will leave his post in in September
"I had in my own mind some ideas about where the bureau needed to go and then a week later we had September 11. I did not expect I would be spending my time preventing terrorist attacks."
During Mr. Mueller's time at the bureau US authorities have had dealt with significant terror threats and attacks, including the underpants flight bomber in 2009 and the cargo plane printer bomb threat in 2010.
He has also dealt with the Boston Marathon bombing and the 2009 shooting that killed 13 and injured more than 30 at Fort Hood in Texas.
Speaking to reporters at the FBI headquarters, he also defended the National Security Agency (NSA) classified surveillance programmes, exposed by whistle-blower Edward Snowden.
He said it was "tremendously important to the protection, not only from terrorist attacks, but from other threats to the United States".
Mr. Mueller made the comments on the same day that a new round of revelations about the surveillance showed that the NSA scooped up as many as 56,000 emails and other communications annually over three years by Americans not connected to terrorism.
His successor, former Justice Department official James Comey, will be on hand next week for the transition.
