In the immediate aftermath of the terrorist bombings in Brussels conducted by radical Islamists, authorities reportedly evacuated non-essential staff from the nearby Tihange nuclear-power plant. At the time, the moved raised a few eyebrows but mostly got lost in a deluge of reporting on the attacks themselves. A new report from NBC News provides a connection between the plotters of the ISIS attacks and the nuclear infrastructure in Belgium, one that might have made authorities fear another kind of attack altogether:

The brothers behind this week’s Brussels bombings also spied on a top nuclear researcher and hoped to build a so-called “dirty bomb,” an expert involved in a probe into ISIS threats told NBC News on Thursday.

Khalid and Ibrahim El Bakraoui were responsible for planting a hidden camera outside the Belgian researcher’s house, according to Claude Moniquet, a French former intelligence official who was hired to investigate potential plots targeting Europe’s nuclear sector.

This camera produced more than 10 hours of film showing the comings and goings of senior researcher at a Belgian nuclear center and his family — footage that was seized during a Belgian raid in November, officials announced last month.

They discovered the footage in November as part of the investigation into the Paris attacks, and according to CBS, Belgian officials hd connected the Bakraoui brothers to the surveillance by December. That kind of surveillance wouldn’t lend itself directly to an attack on a facility, but it certainly could have allowed the Bakraoui brothers an opportunity to seize the researcher, or his family. That is what investigators believe they intended — a blackmail scheme that would have given them access to nuclear material as a means to build a dirty bomb. They could also have forced the researcher to infiltrate them into a nuclear facility by holding his family hostage, a scenario that occasionally plays out in robberies of jewelry stores, banks, and other secured facilities.

Even if the plot didn’t involve a direct attack on the facility itself, its existence raises the possibility that other such attempts to snare key personnel may have succeeded. The replacement of non-essential personnel with security forces was an attempt to pre-empt any follow-up attacks:

These facilities would be operating at this level of reduced staffing for the foreseeable future to ensure no unauthorized personnel could gain access, a spokesman for Belgium’s Federal Agency for Nuclear Control (FANC) told Reuters on Tuesday.

That plot was uncovered in February, so when the bombings began, it made sense for FANC to act quickly to reduce the threat profile at its nuclear power plants.

Perhaps, though, it would have also made sense for Belgian officials to keep the Bakraoui brothers in prison — where they have spent considerable time. In fact, both of them should still be in prison:

Carjackings, robberies and shoot-outs with police were just some of the convictions collected by the Belgian brothers who took part in the metro and airport assaults in Brussels claimed by the Islamic State group. …

Baby-faced Ibrahim, 29, who blew himself up at the airport on Tuesday along with Laachraoui, had been given a nine-year sentence in 2010 after a gunfight with police, according to local media. …

His younger brother Khalid, 27, who blew himself at the Maalbeek metro station just a short walk from the main EU institutions, was a convicted carjacker, receiving a five-year sentence 2011, according to La Derniere Heure.

Turkey tried to warn Belgium of Ibrahim’s connections to terrorism last year, according to Turkish prime minister Tayyip Erdogan:

They couldn’t identify a link to terrorism? They certainly had plenty of their own links to violent crime, and kept releasing both Bakraoui brothers anyway. One would think that authorities would have seen that criminal history and combined it with the warning from Turkey to pay more attention, but the Belgians apparently haven’t done well at connecting dots. We had the same problem before 9/11 and since (the Tsarnaev brothers come to mind as especially analogous), so it’s not as though it’s an unknown failure in the West, but the lack of action against the Bakraouis after discovering the surveillance on the nuclear researcher seems like a particularly indefensible failure.