Current and former U.S. and European counter-terrorism officials told NBC News that the location and timing of the attacks -- just days after the capture of the suspected leader of the Bataclan massacre in Paris -- suggested a level of unpreparedness by Belgian authorities that one ex-official called "shocking."
Another said that Tuesday's attacks would be "a watershed" if they are linked to the same network responsible for Bataclan. "In the past," said Norwegian terror expert Thomas Hegghammer, "nobody got to strike twice."
The officials described Brussels, especially the Molenbeek neighborhood a few miles from 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 Molenbeek, 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."
One European official said that while it may be too early to criticize Belgian authorities, criticism is likely to mount as details emerge.
Hegghammer said the Brussels attacks were troubling because local authorities were already engaged.
"This happened while the authorities were already on high alert and hunting intensively for suspects," said Hegghammer. "We can say they were 'maxed out' on the investigation side, yet this still happened."
Tuesday's killings may signal a shift in the power balance, said Hegghammer, a senior research fellow at the Norwegian Defense Research Establishment, which advises the Ministry of Defense and the armed forces.
"It's an untenable situation, and there will be a lot of pressure on the Belgian services to at least go find the guys who are still on the run," Hegghammer said. "I expect a lot of doors to be kicked in over the coming weeks."