The murder of a security guard at a Belgian nuclear facility just two days after the Brussels attacks, coupled with evidence that Islamic State operatives had been watching researchers there, has re-ignited fears about ISIS and nuclear terrorism. Some experts, including ones cited by the New York Timesand others, dismiss the possibility that ISIS could make even a crude nuclear bomb. But Matthew Bunn, the co-principal investigator at the Project on Managing the Atom at Harvard’s Belfer Center, says that the threat is quite real.

Belgium has seen numerous suspicious events related to nuclear material and facilities. In August 2014, a worker at the Doel-4 nuclear power reactor opened a valve and drained a turbine of lubricant. The valve wasn’t near any nuclear material, but the act caused at least $100 million in damage and perhaps twice that. Later, Belgian authorities discovered that a man named Ilyass Boughalab had left his job at Doel-4 to join the Islamic State in Syria. (His last background check was 2009.)

In November, shortly after the Paris attacks, Belgian authorities arrested a man named Mohammed Bakkali and discovered that he had video surveillance footage of an expert at Belgian’s SCK–CENnuclear research facility in Mol. It now seems that the footage was collected by Ibrahim and Khalid el-Bakraoui, two of the suicide bombers in the recent Brussels attacks.

Then on March 24, a guard was found shot at Belgium’s national radioactive elements institute at Fleurus. A Belgian prosecutor declared the death unrelated to terrorism and denied reports that the guard’s security pass had been stolen and hastily de-activated.

No matter what happened at Fleurus, mounting evidence points to ISIS’s intention to cause nuclear havoc, whether by damaging a nuclear facility, spiking a conventional bomb with radioactive materials, or even building a fission bomb with highly enriched uranium.

The first concern is that sabotage could create a Fukushima-like environment in central Europe. But to pull that off, Bunn writes in a blog post obtained prior to publication by Defense One, militants, criminals or terrorists would need a lot of specialized knowledge of the plant’s security features and measures and how to defeat them.

Just before the most recent attack in Belgium, SCK–CENdeployed armed troops to Belgium’s four nuclear sites.

Dirty Bombs

But beefing up security at explicitly nuclear sites still leaves a lot of radioactive material less well protected. “Radiological materials are available in many locations where they would be much easier to steal, in hospitals, industrial sites, and more,” than at the SCK–CEN center, Bunn wrote. Such materials can allow a terrorist to turn a regular-size blast into a catastrophe that renders an entire area essentially poisonous, greatly increasing the costs of cleanup and the long-term danger to survivors, first responders, etc. In 1987, four people died in the Brazilian city of Goiânia from exposure to cesium salt, derived from junked medical equipment.

Bunn points to a recent report from the Nuclear Threat Initiative, which notes that the material to make a dirty bomb exists in “tens of thousands of radiological sources located in more than 100 countries around the world.”

In 2013 and 2014, there were 325 incidents of radioactive materials being lost, stolen, or in some way unregulated or uncontrolled, according to the report, which cites estimates from the James Martin Center for Nonproliferation.

One material of particular concern is Cesium-137, or Cs-137. A byproduct of fission that’s commonly used in radiation cancer therapy, “it exists in many places much less well protected thanSCK–CEN,” Bunn writes.

The ultimate nightmare takes the form of a nuclear bomb composed of highly enriched uranium. Bunn wrote that stealing highly enriched uranium from SCK–CEN would have been very difficult for the Brussels suicide bombers. And yet, he wrote, “The Times story largely dismissed – wrongly, in my view – the idea that the HEU at SCK–CEN might have been the terrorists’ ultimate objective, saying that the idea that terrorists could get such material and make a crude nuclear bomb ‘seems far-fetched to many experts.’”

Citing a recent Belfer Center report, he wrote, “repeated government studies, in the United States and elsewhere, have concluded that this is not far-fetched.”

One key passage in the report offered this insight: According to the International Atomic Energy Agency, 13 incidents of the “illegal possession, sale, or movement” of highly enriched uranium occurred between 1993-2014. None of those involved material over a kilogram, not nearly enough to build a nuclear bomb. But “incidents involving attempts to sell nuclear or other radioactive material indicate that there is a perceived demand for such material. The number of successful transactions is not known and therefore it is difficult to accurately characterize an ‘illicit nuclear market.’”

It’s hard to tell how successful an assault on a facility like SCK–CEN would be if attempted by two lone gunmen, even if they had kidnapped an expert. But ISIS’s attraction to nuclear material, and perhaps even a nuclear bomb, seems to be growing.