Searches in the wake of last year's deadly terror attacks in Paris have uncovered video linked to a person working in Belgium's nuclear industry.
The Belgian federal prosecutor's office said in a statement on Wednesday that an "investigation is actively ongoing".
Belgian media reported that several hours of video was filmed near the home of a man thought to be a senior official at the Mol nuclear waste facility in the Flanders region.
The house of the unidentified man was apparently filmed from a nearby wood.
Prosecutors declined to confirm that information or provide details.
Security services have long feared extremists might get their hands on radioactive material.
It comes as a cloud hangs over some of Belgium's nuclear plants after a number of unplanned shutdowns.