Determining these connections is a security imperative as U.S. and European counterterrorism and intelligence officials strongly believe there are other plots still in the making. Some officials believe it took months to plan with coordination from Syria, including CIA Director John Brennan, who said Monday, "This is not something that was done in the matter of days. This was something that was deliberately and carefully planned over the course, I think, of several months in terms of making sure they have the operative, the weapons, the explosives, the suicide belts."
Other officials say there are signs the attacks were planned more recently in Belgium, noting the plot involved a relatively large number of people but seems to have managed to stay under the radar, something that becomes increasingly more difficult to do the longer the planning continues.
The idea that the plot could have been planned and coordinated locally with little notice is worrying to law enforcement, as it would mean the Paris plot could represent a new model for attacks in the West that are exceedingly difficult to thwart. One U.S. counterterrorism official described it as a "low-tech, high-impact" plot" that may have been inspired or encouraged by the ISIS core but plotted locally.
Officials stress that the investigation is still in its earliest stages and more information on the plot has yet to be uncovered.
Investigative efforts to unravel the plot are being slowed by the fact the terrorists appeared to work hard to cover their tracks, counterterrorism and intelligence officials tell CNN. The officials say no martyrdom videos so far have been published featuring the attackers. Family members interviewed so far have not described any effort to say goodbye or indications in advance that an attack could be coming.
As CNN first reported, cell phones found on the scenes of the attack contained encryption apps that hid any messages and identities of who received them, according to the sources.
The suicide vests laden with the homemade explosive TATP are described as relatively unsophisticated and could have been based on designs found on the Internet. French investigators managed to recover one vest intact and are examining its makeup.
Investigators have begun putting together a picture of the attackers, finding links among them. Some were roommates, others were family members.
One theory French investigators are pursuing is whether the suicide attacks on the Stade de France were simply a diversion designed to draw police and give the other attackers room to carry out their shootings on restaurants and at the Bataclan concert hall. No conclusions have been made.
The plotters would have known the venue had high security, investigators believe, and there was less chance the bombers could get inside. One of the attackers tried to enter the stadium but was rebuffed by security who discovered the suicide belt he was wearing.