Yesterday we learned this from FBI Director Robert Mueller's testimony before a House appropriations committee.
“Terrorists have not used the Internet to launch a full-scale cyber attack, but we cannot underestimate their intent.”
He also said terrorists have shown interest in developing hacking skills, and that the evolving nature of the problem makes the FBI’s counterterrorism mission more difficult.
So terrorists showing an interest in developing hacking skills--What, did they google "hacking skills"?--are more capable than Anonymous and LulzSec?
He could make just as much sense by claiming that a 15-year-old kid who's interested in driving can win the Indianapolis 500 his first time behind the wheel. We wouldn't want to underestimate the kid's intent.