WASHINGTON -- The Islamic State has broken away from al-Qaida's highly centralized approach to terrorism, opting instead for a more flexible, opportunistic model for attacks in Europe that gives operatives more autonomy and presents fewer opportunities for Western security services to thwart plots, current and former U.S. officials say.
While investigators are still chasing leads in the Brussels bombings, those attacks and the earlier rampage in Paris have provided greater insight into how the Islamic State runs its external operations, how its approach to terrorism differs from al-Qaida's and what opportunities and challenges that presents for combating the group.
With the fight against the Islamic State show no signs of slowing down, President Barack Obama is to meet with his national security team Wednesday at CIA headquarters to discuss the administration's strategy.
The Islamic State, which grew out of al-Qaida's branch in Iraq, has eclipsed its parent organization to become the preeminent global jihadi group since it declared its caliphate in mid-2014 in territory it controls in Syria and Iraq. It now presents a more formidable adversary to Western counterterrorism efforts, in part because the Islamic State has adapted its terrorism template based off of what worked and failed for al-Qaida.
The differences start with the role of the central leadership. Abu Mohammed al-Adnani, a Syrian who was once held by U.S. forces in Iraq, is the head of what CIA Director John O. Brennan has called the Islamic State's external operations branch.
Al-Adnani is not viewed as a mastermind who micromanages plots in Europe -- although he is understood to be more hands-on with operations in the Gulf and Lebanon -- but rather as a facilitator who grants operatives the power to take the initiative, experts say
Individuals appear to be selected to mount attacks in Europe, at least in part, based on their language skills and nationality. They then filter back into Europe to carry out an attack and are given a degree of latitude -- just how much is still a matter of debate within counterterrorism circles -- on timing and targets.
"When it comes to external attacks, ISIL's pliable nature means that there are fewer opportunities to discover elements of the plots," a U.S. counterterrorism official told CQ on condition of anonymity in order to discuss efforts to combat the Islamic State. "This is, in part, because some plots themselves are being hatched and executed by operatives already sent abroad. The lack of rigid assignments within ISIL also means that understanding plots can be challenging because the roles of various parts can change."
Some current and former officials say Islamic State commanders give general guidelines -- almost "we want something like this that will have an impact" -- and the cell is then allowed to decide more or less how to achieve the desired result.
Sen. Richard M. Burr, the chairman of the Senate Intelligence Committee, told CQ that Islamic State militants seem to have a range of options for targets and "there's a bit of latitude almost that the operational cells could choose from that basket of things."
Al-Qaida, in contrast, tended to organize and direct plots from the group's central leadership. An individual would travel to the tribal areas along the Afghanistan-Pakistan border, where he would receive training and instructions from al-Qaida's central leadership before being sent back out into the world to conduct an attack.
John McLaughlin, a former deputy director and acting director of the CIA who retired in 2005, said the "improvisational character" of the Islamic State's operations -- something officials rarely saw with al-Qaida -- makes thwarting plots and dismantling networks "more difficult."
Al-Qaida's strong central control meant that there were often several specific elements to a plot -- date, target, operatives -- that the United States or its allies could uncover and work off of to try to thwart the plan. If the United States knew what the target was, it could take defensive measures, for instance.
The flexibility of the Islamic State's attack model makes that harder to do.
"You could pull on the end of (an al-Qaida plot) thread and start to unravel it," the U.S. counterterrorism official said. "With ISIL, you can pull on threads but (they) may not lead anywhere because sometimes, until the last minute, there is no date, there may be no place and not all the people have been integrated into the plot."
That lack of a set target also makes it difficult to predict what the Islamic State will try to hit, since seemingly anything and everything is judged fair game.
"You don't know what their targeting mindset is going to be," said Clint Watts, a former FBI counterterrorism agent who is now a Fox Fellow at the Foreign Policy Research Institute. "We could kind of think through al-Qaida and say 'Ah, they're not going to hit a shopping mall because they're actually kind of worried about women and civilian casualties, or it's not a symbolic target.'"
With the Islamic State, he said, "it's harder to anticipate the targets."
Several factors allow the extremist group to operate with this amount of flexibility.
First, the Islamic State has a large number of people it can pull from for its operations. With the help of its aggressive online recruiting and battlefield success, the group has attracted tens of thousands of foreign fighters, some 5,000 of whom have Western passports. That gives the group a large pool of individuals with EU identity papers to draw from for operations in Europe -- something al-Qaida sought but never had.
"The access problem is different than it was with al-Qaida, where they just simply didn't have that many people with Western passports -- that was a big deal for them," McLaughlin said. "Here you've got a large number of Western passports and people who can move in Schengen and people who can come to the United States without a visa requirement."
The Islamic State also has a network of sympathetic individuals who haven't fought in Syria and Iraq but who are ready to help support a plot, such as Ibrahim and Khalid el-Bakraoui, two Belgian brothers who blew themselves up in Brussels.
European-born or raised Islamic State operatives also can easily blend back in to the cities and neighborhoods they came from in a way that al-Qaida operatives often could not.
"They can move so much quicker because they're plotting, planning at home, they've got a support network like al-Qaida guys didn't have in these countries," Watts said.
The people who have fought in Syria and Iraq and returned to Europe have come back trained and battle-hardened, with experience using small arms and explosives.
"They're using explosives that al-Qaida couldn't operate because the Islamic State's operatives just have so much battlefield experience. They've cooked TATP a dozen times," Watts said, referring to the kind of homemade explosives used in the Paris and Brussels attacks.
All told, it presents a formidable new challenge for Western intelligence services to try to disrupt and dismantle such networks.
"I think European services are dealing with a volume of work and a volume of potential extremists that exceeds anything al-Qaida was ever able to field," McLaughlin said. "In al-Qaida's successful attacks in London and Madrid after 9/11, there were a handful of people involved. They did succeed to a degree, but those were very isolated attacks and we didn't see the potential for them repeating over and over again."
The Islamic State has already demonstrated the ability to follow up on the bloodshed in Paris with another round in Brussels, and investigators are scrambling to try to roll up the network before another attack can take place.
Belgian authorities on Friday arrested Mohamed Abrini, who has confessed to being a third attacker at the airport. Abrini was caught on surveillance video wearing a hat and walking with the two other suicide bombers. Explosives in Abrini's bag did not detonate.
As much of a challenge as the Islamic State's approach to terrorism in Europe poses, it is not without its vulnerabilities.
Many of the group's operatives are known to law enforcement -- although not as terrorism suspects -- because of previous criminal activity, the counterterrorism official said. That means that authorities may have information on the militants themselves, their family and their friends.
"Because these networks overlap or resemble criminal networks, it is a model the law enforcement agencies and interior ministries are familiar with," the official said.