U.S. intelligence officials believe that the elder of the two French suspects, Said Kouachi, received terrorist training from al Qaeda's Yemeni affiliate for a couple of months in 2011, with the idea of him returning home to mount an attack.

French authorities knew Kouachi traveled to Yemen, but it's not clear whether they knew what he did there, according to an intelligence assessment described to The Associated Press, Still, French authorities placed both Kouachi brothers under close surveillance when he returned.

U.S. officials believe the brothers led a normal life for long enough that the French began to view them as less of a threat and reduced the surveillance.