Le Soir newspaper said the man was a guard at Belgium's national radioactive elements institute at Fleurus, to the south of Brussels.

A police spokeswoman said she could not comment because an investigation was ongoing.

In a nation on high alert following this week's attacks, the media reports tap into fears about the possibility militants are seeking to get hold of nuclear material or planning to attack a nuclear site.

On Thursday, DH had reported the suicide bombers who blew themselves up on Tuesday originally considered targeting a nuclear site, but a series of arrests of suspect militants forced them to speed up their plans and instead switch focus to the Belgian capital.

Late last year, investigators found a video tracking the movements of a man linked to the country's nuclear industry during a search of a flat as part of investigations into the Islamist militant attack on Paris on Nov. 13 that killed 130 people.