The full version of this story was originally published by The Center for Public Integrity, a nonprofit, nonpartisan investigative news organization in Washington, D.C.

A small video camera stashed in a row of bushes silently recorded the comings and goings of the family of a Belgian man with an important scientific pedigree last year, producing a detailed chronology of the family’s movements. At one point, two men came under cover of darkness to retrieve the camera, before driving away with their headlights off, a separate surveillance camera in the area revealed later.

The Belgian police discovered the secret film on Nov. 30 while searching the home of a man with ties to ISIS. But they became far more alarmed when they figured out that its star was a senior researcher at a Belgian nuclear center that produces a significant portion of the world’s supply of radioisotopes.

That realization quickly got the attention of the world’s counterterrorism experts.

A diagnostic tool used by hospitals and factories around the globe, radioisotopes are also capable of causing radiation poisoning and sickness, making them a potential target for terrorists seeking to build a so-called “dirty” bomb that could contaminate the downtown area of a major city, sowing panic and causing billions of dollars in financial losses.

This map, based on an atmospheric dispersion model, shows where individuals are projected to have an increased risk of developing cancers due to radiation exposure over a year or more. PAGs are primarily based on an assessment of the risk of developing cancer over an exposed individual’s lifetime. Congressional Research Service and William Rhodes III, Senior Manager, International Security Systems Group, Sandia National Laboratories, September 2010; analysis by Heather Pennington; graphics by Mona Aragon.

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Mohamed Bakkali, who rented the home where the films were seized in a raid, was captured on Nov. 26 and has been charged with engaging in terrorist activity and murder stemming from his alleged involvement in the Nov. 13, 2015 siege in Paris that killed 130 people and wounded hundreds more.

Belgian authorities have since speculated that the group was trying to figure out a way to collect materials from the nuclear center as the first step in building a bomb.

“We can imagine that the terrorists might want to kidnap someone or kidnap his family,” so they can force their target to turn over the radioactive innards of such a device after removing the materials surreptitiously, said Nele Scheerlinck, a spokeswoman for Belgium’s Federal Agency for Nuclear Control, the nation’s nuclear regulator.

Click here to read the Center for Public Integrity's full version of this story.

Many U.S. experts consider the eventual detonation by terrorists of a dirty bomb containing radiological materials to be inevitable.

“We know that it would not require a team of nuclear physicists or even a particularly sophisticated criminal network to turn raw material into a deadly weapon,” an internal Energy Department report on the threat, designated “Official Use Only,” declared in May 2013. “In many cases, a determined lone wolf or a disgruntled insider is all it might take.”

But until now, there has been no public, concrete evidence that a particular terrorist organization is aggressively pursuing the radioactive building blocks of a dirty bomb. Experts have noted that such materials are too plentiful to count precisely, but roughly estimate they are contained in more than 70,000 devices, located in at least 13,000 buildings all over the world — in many cases without special security safeguards.

“The potential for a bad outcome when you have ISIS looking at nuclear people is substantial,” said William H. Tobey, a former deputy administrator for defense nuclear nonproliferation at the National Nuclear Security Administration. Several other experts said they are particularly alarmed that the incident occurred in Belgium, which they say has a troubled record on nuclear safety issues.

The researcher in question, who has not been identified, was sufficiently senior to have had broad access to sensitive sites throughout the research center. Officials declined to say if he is now under police protection.

Scheerlinck added that in the view of her colleagues, whatever was being planned would have failed, because the nation’s radiological materials are under tight control. But other Western experts are not so sure, given their belief that Belgian nuclear authorities have historically been too casual about security risks.

The Center for Public Integrity is a nonprofit, nonpartisan investigative news organization in Washington, D.C.