The brothers planted a camera in bushes outside the home of the research and development head of the Belgian nuclear program, NBC News reported.
The two were seen in December recovering the camera and leaving in a car with its lights turned off, the Derniere Heure newspaper reported.
The 10 hours of footage, which included images of a local bus, was discovered in the Brussels home of another ISIS suspect that police raided after the Nov. 13 attacks in Paris.
Claude Moniquet, a French former intelligence official investigating potential plots targeting Europe’s nuclear sector, told NBC News that the El Bakraoui brothers were behind the failed plot.
“The terrorist cell … naively believed they could use him to penetrate a lab to obtain nuclear material to make a dirty bomb,” said Moniquet, CEO of the European Strategic Intelligence and Security Center private consultancy.
After Tuesday’s attacks in Brussels, authorities evacuated Belgium’s Tihanghe nuclear plant, where 11 workers have had their access revoked since last week, Belgian state broadcaster RTBF reported Thursday.
The Tihange and Doel plants will operate with reduced staffing for the foreseeable future, a spokesman for Belgium’s Federal Agency for Nuclear Control told Reuters.
When authorities made the chilling discovery in February, 140 soldiers were posted at nuke sites around the country — possibly leading the jihadists to change course and focus on softer targets, like subways and airports.
The same ISIS cell is believed to have been responsible for the attacks in Paris and Brussels.
“This is a network orchestrated by young Belgians, most of whom went to Syria where they learned to handle weapons and explosives,” DH reported.