A former al Qaeda associate who spied on the group for Western intelligence agencies said that the men responsible for the Charlie Hebdo massacre could have been a “sleeper cell,” that spent years preparing for the attack.

“What I can say from my experience ... is that these people here have managed to deceive French intelligence to believe that, while they were once extremists, they no longer were,” Morten Storm, who was affiliated with al Qaeda in the early 2000s before spying on the organization for Western governments, told ABC News. “They managed to get under the radar and ... they finally woke up again, like a sleeper cell, and [committed] this atrocity.”

Storm said he believed that those responsible for the attacks in Paris could have been one of many such al Qaeda cells in position in Europe, preparing to carry out attacks, according to Fox News.

Members of al Qaeda in the Arabian Peninsula (AQAP), the group's Yemeni branch, reportedly claimed responsibility for the attacks in Paris on Friday. A member of the group told the Associated Press that it had directed the attacks “as revenge for the honor,” of the Prophet Muhammad.

Storm told ABC that Anwar-Al Awaki, a high-ranking member of AQAP, had asked him to steer “European brothers” to the organization, so that they could be trained to carry out attacks and return to Europe without arousing suspicion.

The men believed responsible for the attack on Charlie Hebdo, Chérif and Saïd Kouachi, were killed in a shoot-out with police on Friday. Saïd Kouachi spent time in Yemen in 2011 receiving military training from the group, and Chérif Kouachi previously told a French TV station that al Qaeda had sent the brothers to commit the attack.

The U.S. State Department updated its travel advices for Americans traveling abroad in the wake of the attacks. It warned that, while it had no intelligence of specific threats, Americans anywhere in the world could be targeted for kidnapping or terrorist attack, the Washington Times reported.