While normal wiretaps and mobile phone surveillance can be done by small intelligence and police services such as those in Belgium, grabbing huge amounts of phone data and electronic signal intelligence — and rapidly processing it — was beyond their capabilities.
The Belgian authorities knew they needed help, and had made a decision, which has not been previously reported, to involve an ally with a vested interest in dismantling a dangerous ISIS network: They called on the US National Security Agency (NSA).
The two officials described the scene at the funeral, where a known suspect was filming on his cell phone: “The guy is filming on a smartphone — that tells us he’s going to send that file to someone, right?” the security service source said. “We had the NSA hit that phone very hard.”
The NSA refused to comment on the operation, but a spokesman for the Director of National Intelligence forwarded an article in which James Clapper said: “The NATO Alliance faces an increasingly complex, diffuse threat environment. Consequently, we are always striving toward more integrated intelligence to stay a step ahead.”
On March 15, just a few days after the funeral, Belgian police made a move based on the information they had garnered from the NSA. Alongside French investigators, they raided an apartment in the Brussels neighborhood of Forest. It ended in a firefight; four officers were wounded and one of the occupants was killed. But investigators learned from fingerprint and DNA evidence that Abdeslam and a co-conspirator, Mohamed Abrini, had been there, although the two men escaped over city rooftops during the shoot-out.
It was an embarrassing blow to the investigation, but the NSA was at least now helping the Belgians track the suspects via their phones. Having lost his safe house, Abdeslam was forced to move around and communicate with people outside his rapidly shrinking network. Abdeslam and Abrini called a friend searching for a new place to hide out.
That’s when, according to the military intelligence official, they got him: “Finally … we have this asshole.”
If you polled Europeans on whether it was right for the Belgian authorities to enlist the help of the supposedly infamous and hated NSA to catch a terrorist fugitive, 70-80% would say 'yes'. The number would probably be even higher among French and Belgian people.
Ordinary people have no problem with their communications being monitored, as long as (1) they don't know it's happening and no abuses come to light; and (2) the authorities can claim some legitimate purpose for doing so. You may find this apathy reassuring, you may find it appalling (this is not a normative argument about whether surveillance is good or bad), but it is the case.
If I were designing a remedial training course for journalists, one of the key lessons would be to always, always perform an independent check to see if your subject's revealed preferences line up with their stated preferences. Even if your subject is (what you consider to be an) underdog. Especially if he's an underdog.