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With no one immediately claiming responsibility for Friday night's terror attacks in Paris, intelligence officials will have to piece together a myriad of clues.
That process could take a while, according to the former director of the National Counterterrorism Center.
It's too early to say if the killers were linked to al Qaeda or ISIS or another group, Michael Leiter told NBC News.
However, the attacks show "a level of sophistication we really haven't seen in an urban area since 2008 in the attack in Mumbai, India," Leiter said.
In those attacks, coordinated gun and grenade assaults on multiple sites left 166 people dead, including six Americans.
"This will be a game-changer for how the West looks at this threat," Leiter said.
Meanwhile, a senior U.S. intelligence and counter-terrorism official told NBC News that the Paris attacks, because of their sophistication, didn't appear to be the work of ISIS. Rather, the level of coordination points more toward al Qaeda, the official said.
But not long after reports emerged of gunshots outside a Paris restaurant followed by other near-simultaneous incidents, ISIS supporters and members took to Twitter to celebrate the attacks.
Many used the Arabic hashtag: "Paris_is_burning."
Global security firm and NBC News consultant Flashpoint Intelligence pointed out early Saturday that while ISIS hadn't claimed responsibility for the attacks, its "fighters, supporters, and unofficial mouthpieces have embraced the operations in a much more rapid and aggressive manner than any other group or adherents to alternate ideologies."