Current and former U.S. and European counter-terrorism officials told NBC News that Tuesday's deadly attacks in Brussels raise questions about the ability of Belgian authorities to cope with the scale of the country's terror problem.
A senior Justice Department official said that Belgian authorities have been open about their frustrations in keeping up with terrorists, since "they know they're sitting on a time bomb."
"They've been so proactive," he said. "They have spent a lot of time and money bolstering their infrastructure, but they know it's not enough."
The attacks -- which came just days after the capture of the suspected leader of the Bataclan massacre -- would be "a watershed" if they are linked to the same network responsible for Bataclan, said Norwegian terror expert Thomas Hegghammer. "In the past, nobody got to strike twice."
A senior U.S. counterterrorism official said that Belgians had "yet to up their game commensurate with the risk they are facing."
U.S. and French officials told NBC News that in their view, Belgian authorities have not been aggressive enough in pursuing terror suspects in their territories since the Bataclan attack in Paris, which was staged from Belgium.
Belgian security services do a poor job of sharing intelligence among themselves, officials say, and they are hamstrung by restrictive laws, such as one that bans police from mounting night raids in private homes.
Salah Abdeslam, a suspect in the Paris attacks, was able to hide out in Belgium, apparently in the Brussels neighborhood of Molenbeek, for 125 days before he was captured last week.
One intelligence source told NBC that Belgian authorities were aware an attack was coming, and were closing on the plotters, but obviously did not take action in time.
The officials described Brussels, especially the Molenbeek neighborhood, 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 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."
A senior Justice Department official said that Belgian authorities had been open about their frustrations in keeping up with terrorists, since "they know they're sitting on a time bomb."
"They've been so proactive," he said. "They have a lot of time and money bolstering their infrastructure and resilience, but they know it's not enough."
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."
Frank J. Cilluffo, a former senior U.S. counter-terrorism official, said that Brussels is now the "Ground Zero" for jihadism in Europe.
"The fact that [Abdeslam] was able to evade authorities for so long demonstrates the high level of support for their network in the community," said Cilluffo.
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."