A combination of electronic intercepts, human sources and database tracking indicates several possible targets had been picked out by the ISIS operatives over the last few months since the Paris attacks, according to U.S. counterterrorism officials.
Since the attacks earlier this week, information found in a raided apartment, including maps, indicated other potential targets had been chosen, one of the sources said.
According to a senior Belgian counter-terrorism official, investigators believe the Brussels ISIS cell were composed of two teams who were planning a larger attack or series of attacks in Belgium at a later date. After police discovered Salah Abdeslam's hiding place on Tuesday last week, investigators believe the second team, including suspected bomb-maker Najim Laachraoui, accelerated their timetable. They believe the second team consisted of the Bakraoui brothers and at least two others now on the run.
Investigators believe the first team was composed of Salah Abdeslam, a still-to-be identified accomplice taken into custody last Friday at the same time and Mohammed Belkaid, the suspected commander of the Paris and Brussels attack cell, according to the official.
Belkaid was shot dead during the firefight at Abdeslam's hiding place last week after Belkaid provided covering fire so Abdeslam could flee the building, the Belgian source said. Police recovered an ISIS flag, a Kalashnikov, detonators, and ammunition at the safehouse, suggesting that Belkaid and Abdeslam's team were planning gun and bomb attacks in Brussels like the same network had carried out in Paris. An ISIS flag was also recovered at the cell's bomb factory in Schaerbeek, where the airport attackers set off from early Tuesday morning to launch their attacks.
The office of the Belgium's prime minister would not answer CNN queries about the information.
Officials fear there are dozens of potential and known ISIS operatives still in Europe they're concerned about and some of them may overlap with the Paris and Brussels attackers. One of the sources says plots haven't been disrupted either because the information about the plots is too fragmentary or because investigators are trying to gather more intelligence. The source also stressed that the situation with the plots is very fluid and can change very quickly depending on the opportunity for the operatives some of whom have been trained by ISIS in Syria and Iraq.
Another official cautioned that there are multiple threat streams but not all get verified.
ISIS has released a video compiled using the group's file footage, urging its fighters in the West to carry out more attacks.
"Intel doesn't come in a neat form answering the who, what, where, when and why," the official said. "We rarely are operating with a full utility belt."
Other U.S. counterterrorism officials say there was chatter before the Brussels attacks indicating something was about to happen, but nothing specific enough to indicate where and when the attacks were going to happen.
"There is not that calm in between the storms we used to see," said one U.S. official, who said typically one would expect there would be some time after an attack where others regrouped.
Officials say some of the other attacks they have been concerned before with specific times and places picked out did not pan out, but the sources cautioned ISIS followers are opportunistic and flexible if a planned attack doesn't get executed.
"There's evidence these guys are floating around in Europe and haven't been rounded up yet and hope to launch an attack," one official said, adding, "there's a constant drumbeat and fear something else is going to happen."
The concern was reflected in a State Department alert warning U.S. citizens traveling to Europe that terrorists continue to plot near term attacks.
It reflected a wider concern about the growing number of ISIS devotees who have been to Syria and Iraq and since returned. Ahead of the Paris attacks, Western intelligence received fragmentary information that ISIS external operations division dispatched 60 operatives to hit five cities, including Paris, London, Berlin, a major city in Belgium and another city. Paris mastermind Abdelhamid Abaaoud boasted of some 90 ISIS operatives already in Europe.
On Wednesday, the top Democrat on the House Intelligence Committee said there are a "large number" of such fighters back in Europe.
"Potentially hundreds," said Rep. Adam Schiff, D-California, in an interview that aired on "The Situation Room" with CNN's Wolf Blitzer. "If you include both foreign fighters that have returned from the fight as well as people that didn't leave for the fight but in communication ... with ISIS, it could well reach those numbers."
The director of the FBI said Thursday that ISIS adherents returning to Europe are a continued concern.
"For almost three years now were very much worried about people around the glove we worry who travel to Syria and gets the worst kind of training and then flow back out, and it's especially a problem in Europe. And so, this is a manifestation of that problem and I don't think and doesn't reflect greater skill or organization on ISIS's part," Director James Comey said.