Just as word has come, multiple that plotted to get their hands on nuclear material for a dirty bomb—and they set their eyes on a top Belgian nuclear researcher to do it.

and other outlets report the brothers were actively spying on the researcher, going as far as planting cameras in the bushes outside their house and recording their daily life patterns.

The scientist the brothers had targeted is supposedly one of the top-ranking researchers at the SCKCEN nuclear center in Mol, located about an hour’s drive from Brussels. The facility is one of the world’s largest suppliers of radioactive isotopes for commercial and research purposes, just the type of material that would be useful in constructing a dirty bomb.

The surveillance footage of the scientist’s home was seized last November during a counter-terrorism raid following the attacks on Paris. It is not clear exactly what caused the connection between the brothers and the nuclear plot to become clear just now, although it is very possible that intelligence collected at the brothers’ flat allowed law enforcement to connect the dots. This revelation is also likely why Belgium officials put nuclear sites on particularly high alert following Tuesday’s attack.

it is suspected that the brothers were likely plotting to either abduct the scientist outright or take their family members hostage and make them provide the nuclear material in exchange for the lives of their loved ones.

Dirty bombs—more technically known as radiological dispersion devices—are explosive devices laced with nuclear material. These weapons have been a top fear of terrorism fighting governments around the globe for decades. Unlike a fission or fusion nuclear bomb where heat and shock-waves are the primary method of destruction, a dirty bomb is meant to irradiate a large area and everyone near and immediately downwind of its detonation.

Depending on the device, and especially the nuclear material used in it, the area surrounding the blast-zone can be radioactive for years following the initial detonation.

What makes these weapons especially dangerous is that they are vastly simpler to construct than an actual nuclear bomb and can be built even with commercial nuclear materials found at hospitals and research labs all over the world.

This story is just a reminder of the, especially the radioactive type. Although the thought of nuclear warheads and weapons-grade nuclear material falling into the wrong hands makes headlines, the much lower hanging fruit for terror groups is radioactive materials found in virtually every major city around the globe.

ISIS likely already has the material needed to build dirty bombs. After quickly sacking Mosul last June, one of the prizes the group likely looted . Although there are far more favorable nuclear materials to built a dirty bomb with, this material could be used to create radiological weapons., and ISIS is likely a prime buyer, but that’s a whole other story.)

This all could make more complicated. A dirty bomb is very much an area denial device and ISIS will not give up such a massive strategic win like Mosul easily. Their mindset may be that if they can’t have it they won’t let anyone else have it either. Dirty bombs are nearly perfect devices for fulling such a dark strategy.

Twenty-two months ago, ISIS seized Mosul, Iraq’s second largest city, during a blitzkrieg across…