Current and former U.S. and European counter-terrorism officials, who are experts on ISIS in Europe, told NBC News that the location and timing of the attacks — just days after the capture of the suspected operational leader of the Bataclan massacre in Paris — suggested a "shocking" level of unpreparedness by Belgian authorities.
They described Brussels, especially the Maelbeek neighborhood near the site of the subway strike, as an explosive mix of highly capable foreign fighters trained by ISIS and sympathetic locals who are unknown to authorities but eager to help in attacks.
They noted that when Bataclan terror suspect Salah Abdeslam was arrested last week, authorities found a huge cache of weapons, prompting Belgian Foreign Minister Didier Reynders to warn of other imminent attacks.
"He was ready to restart something in Brussels," Reynders said at the German Marshall Fund's Brussels Forum. Besides discovering "a lot of weapons, heavy weapons," Reynders said, "we have found a new network around him in Brussels."
Clint Watts, a former FBI and U.S. Army counter-terrorism official and expert on how ISIS operates, told NBC News that Belgian authorities should have been more prepared for Tuesday's attacks.
"That they could sit for four months, not only in Belgium but in Brussels and especially in Maelbeek, and plot these kinds of attacks just four days after the arrest of such a high-level network facilitator — this is shocking to me because they should have been on the highest level of alert," Watts told NBC News.
"It is hard to conceive that this would happen on such a large scale when it was so obvious that these guys were operating there," Watts said of ISIS. "After [Abdeslam's] arrest, you would have to assume everyone in the network was preparing to launch whatever they had."
"After the Paris attacks, it was a question of not being able to run all the leads down," Watts said. After Tuesday, "It's no longer a capacity problem, it's a competency problem."
Frank J. Cilluffo, a former senior U.S. counter-terrorism official, agreed.
"It is in essence the Ground Zero of European jihadism, there is no question about that," Cilluffo said of the part of Brussels where some of the attacks Tuesday occurred. "And the fact that [Abdeslam] was able to evade authorities for so long demonstrates the high level of support for their network in the community."
Whether police and intelligence agencies were prepared "is the legitimate question to ask the Belgian authorities," said Cilluffo, who directs the Center for Cyber and Homeland Security at The George Washington University in Washington, D.C. "Obviously, you have to worry about the rest of Europe, hardening those targets right now — notably transportation and places where there are large gatherings."