With a variety of different takes on these events swirling in recent news stories (seehere and here), it’s worth clarifying what we know and what is still unknown about the scale of this threat and how best Belgium — and the rest of Europe — can protect itself. (A just-released Harvard study also has details up through February.)

The short answer is that we don’t know yet — though sustained monitoring of a nuclear expert may be the most troubling indicator yet of nuclear intent from the so-called Islamic State. Belgian terrorists recorded about 10 hours of video of a nuclear official at Belgium’s SCK-CEN research facility, near the town of Mol. As theTimes story reports, Belgian authorities believe the hidden video camera was picked up by Ibrahim and Khalid el-Bakraoui, brothers who are believed to have later been suicide bombers in the Brussels attacks. They reportedly delivered it to Mohammed Bakkali, now under arrest, who is accused of helping with logistics for the Paris attacks……….

If the Belgian suicide bombers were the ones monitoring the nuclear official, it’s possible they first planned to attack the country’s nuclear infrastructure. They may have shifted to the airport when their plans were accelerated by the arrests of co-conspirators, or because of Belgium’s deployment of armed troops to guard its nuclear facilities.

Press accounts of the possibility the terrorists were planning some kind of an attack on nuclear facilities have unduly played down the potential dangers of reactor sabotage. A story in The New York Times, for example, quotes an argument that the TATP explosive the terrorists were using would not get through the steel pressure vessel of a nuclear reactor.

It is certainly true that to cause a major radioactive release, terrorists would have to understand how to overcome a number of different safety and security systems. Getting into a power plant with a suicide vest of explosives would not be enough. But as Fukushima made clear, cutting off a reactor’s electricity and cooling water can cause a disaster that can provoke widespread panic and cause devastating disruption and economic losses………..

In December 2014, after the August sabotage, Belgium’s Federal Agency for Nuclear Control imposed new regulations substantially strengthening protection against insider threats. Then, after the Brussels airport and subway attacks, Belgium withdrew non-essential personnel from its nuclear facilities. This significantly reduced the number of personnel at these sites, reducing the chance that there might be a malevolent insider among them. They also added new rules on security cameras and use of two-person rule in sensitive areas. Moreover, Belgian authorities reportedly withdrew access passes for at least four workers at nuclear facilities in the days after the attacks.

Following the Paris attacks, the revelation of terrorist rings in Brussels and the monitoring of the Belgian nuclear official, Belgium decided to deploy armed troops to guard its nuclear facilities. (A specialized guard force will be trained and deployed in the coming months.)

Two days after the airport and subway terrorist attacks, with armed guards newly deployed at Belgium’s nuclear facilities, a security guard at one site was murdered. The dead man worked at a radioactive materials facility, not a power reactor. Prosecutors have denied early reports that the guard’s security pass had been stolen, and are playing down any militant link. Still, in a country with one of the world’s lowest murder rates — only about 200 murders occur in Belgium each year — it is a surprising coincidence.