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Could attacks like Paris happen here in the U.S.?

U.S. officials are concerned the tactics used to kill 129 people and injure 352 others in Paris could be used here

WASHINGTON - The FBI is sending agents to help French investigators probing Friday's attacks in Paris. At the same time, U.S. law enforcement is trying to track the terrorists' path to the streets of Paris. U.S. officials are concerned the tactics ISIS used to kill there could be used here.

American officials say the attacks were well organized but not sophisticated, and that the explosives used were not high-tech. However, it is the organizational skill and high death count that have counterterrorism officials worried.

"This is the next stage in ISIS' methodology, capability and reach," said Juan Zarate, a former counterterrorism official and a CBS News consultant.

"The real dangerous and troubling part here ... is that you have individuals who are working in tandem, in some way communicating or coordinating, and those communications, that coordination, and the materiel that they brought with them - suicide vests and heavy weaponry - weren't caught in the French security system," he continued.

Just hours after the attacks in Paris, senior FBI officials ordered agents across the country to step up surveillance of potential ISIS sympathizers in the U.S. Currently there are about 900 cases.

Last month, FBI Director James Comey testified before Congress about the threat.

"The challenge we face is ... finding those needles in a nationwide haystack and assessing where are they on that spectrum between consuming poison to acting on poison, and disrupting them before they act," Comey said.

That is something French investigators weren't able to do in Paris.

"You can't prevent every attack and that's the problem. And when terrorists are trying to go after any soft target, strategic or otherwise, you can't protect all sites at all times," Zarate said. "What we saw in Paris was the terrorists winning on that fateful night."

In many ways what happened Friday was the nightmare scenario for Western intelligence officials. Not only were there mass casualties, but a suicide bomber was able to get close enough to the French president that he had to flee.