BRUSSELS — As a dragnet aimed at Islamic State operatives spiraled across Brussels and into at least five European countries on Friday, the authorities were also focusing on a narrower but increasingly alarming threat: the vulnerability of Belgium’s nuclear installations.

The investigation into this week’s deadly attacks in Brussels has prompted worries that the Islamic State is seeking to attack, infiltrate or sabotage nuclear installations or obtain nuclear or radioactive material. This is especially worrying in a country with a history of security lapses at its nuclear facilities, a weak intelligence apparatus and a deeply rooted terrorist network.

On Friday, the authorities stripped security badges from several workers at one of two plants where all nonessential employees had been sent home hours after the attacks at the Brussels airport and one of the city’s busiest subway stations three days earlier. Video footage of a top official at another Belgian nuclear facility was discovered last year in the apartment of a suspected militant linked to the extremists who unleashed the horror in Paris in November.

Asked on Thursday at a London think tank whether there was a danger of the Islamic State’s obtaining a nuclear weapon, the British defense secretary, Michael Fallon, said that “was a new and emerging threat.”