U.S. intelligence analysts have also been combing through signal intelligence, including email and other communications, and have turned up no communications among the attackers. This is partly why U.S. investigators are increasingly convinced the attackers used encrypted communications to evade French intelligence.
One U.S. official described it as an "educated presumption." Two U.S. officials said that view is based partly on the fact that at least one of the attackers at the Bataclan theater, who French authorities had already identified as radicalized before the rampage, was known in the past to have used popular encrypted apps.
U.S. investigators also say they still don't know definitively whether there are more operatives involved in the attack who may be at large. French officials have told CNN they believe there are.
U.S. officials say the scale of the Paris attack has overwhelmed French investigators. "As you can imagine, it's very -- still very chaotic there to get some of the information," Diego Rodriguez, assistant director for the FBI's New York field office, said Monday. "What we do is when we send folks over there, we -- we send them over there so that they can work in collaboration with our legal attaché, who is working with the local police departments, just like we would do here." Over the weekend, the FBI sent additional agents to France to join the personnel already based in Paris in case assistance was needed, but so far France has not formally asked the FBI to join the investigation.
While none of the attackers are known to have traveled to the U.S., counterterrorism officials say the father of one of the Bataclan theatre attackers Ismael Mostefai visited the U.S. several years ago. While the father and other family members were taken into protective custody by French officials in the wake of the attacks, he hasn't been implicated in the plot. And U.S. officials say so far there's no reason to believe the father's visit is relevant to the current investigation. But, as a precaution, FBI agents are still working to interview anyone he visited while in the U.S. and make sure they know everywhere he went.
A bigger concern among US officials centers on the six attackers who, according to French officials cited by CNN affiliate BFM, are believed to have traveled to Syria. Could any of them have bought a plane ticket to the US to carry out an attack?
French authorities have told CNN that two attackers identified so far were previously known to French intelligence officials for being possibly radicalized or having ties to suspected terrorists. U.S. officials say that if those two tried to travel to the U.S. they would have been flagged as a result, because the U.S. checks names against French security lists.
But the others, with no known security flags in their backgrounds, could possibly have entered the U.S. with passports from countries that participate in the U.S. Visa waiver program. One U.S. official says French and European privacy laws have more stringent restriction on the use of watch lists for their citizens. U.S. law gives authorities more leeway to put people on watch lists.
Another problem is a backlog in some European countries, which have seen thousands of their citizens leave to become foreign fighters in Syria and Iraq. "Someone could go to Syria for training for a week and make his way back in [to Europe] and never make into any database," one counterterrorism official told CNN.
Despite a year-long bombing campaign that the U.S. says has killed thousands of ISIS fighters, U.S. officials say the number of westerners traveling to join the group continues to grow. Some countries, aren't able to keep up tracking those citizens. "They're disappearing and going overseas to get training faster than [European officials] can keep up with it," another U.S. official said. "They're so focused on the ones they already know about who have returned---they're overwhelmed and don't have the resources...it's almost secondary who's leaving."