With no one immediately claiming responsibility for Friday night's terror attacks in Paris, intelligence officials will have to piece together myriad clues.
That process could take a while, Michael Leiter, former director of the National Counterterrorism Center, told NBC News.
It's too early to say if it was Al Qaeda or ISIS or someone else, Leiter said. But it almost certainly will turn out to be some kind of Sunni extremists, he said.
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.