In a string of tweets, the hacker or hackers ask The Guardian's editors if they know they're "participating in violence in Syria," and accuse Al Jazeera of "fabricating another child martyr."

"It's alarming not only because free expression is at risk, but because some of these requests come from countries you might not suspect," says Google's senior policy analyst, Dorothy Chou.

"We can tell our efforts are starting to have an impact because extremists are publicly venting their frustration and asking supporters not to believe everything they read on the internet," Clinton said.

Most Americans who don't currently surf the web have never gone online before and live in an internet-free household, Pew researchers found, according to CNN.