(CNN)U.S. law enforcement and intelligence officials say the attacks in France are a realization of one of their biggest concerns about the threat of terror activity in Western countries.
U.S. official on terror attacks: 'This isn't going to stop'
One senior official told CNN: "We've expected this."
The official says the attacks demonstrate how "the boundaries between all of these affiliates is seemingly breaking down and the threat is metastasizing and turning into a global network."
Both of the Kouachi brothers are believed to have traveled to Yemen, with at least one said to have trained with al Qaeda in the Arabian Peninsula. The third suspect, Amedy Coulibaly, told a French television station before he was killed that he associated with ISIS, though investigators have not corroborated that claim.
U.S. officials say the sophistication, coordination and apparent level of training exhibited by the Paris suspects shows how we are moving from a world of mega terror in the 9/11 era to decentralized autonomous, smaller scale and harder-to-detect operations.
The concern was reflected in Friday's FBI and Department of Homeland Security notice sent to law enforcement agencies across the country, highlighting the tactics and techniques used by the suspects -- indicating some kind of formal training.
Even more troubling to U.S. officials is that nations will likely see more of the same. That sentiment was echoed this week by the head of Britain's MI5, who warned more terrorist attacks are to come.
"This is in perpetuity what we're dealing with. It's like the war on drugs. This isn't going to stop," the U.S. official said.