The brothers behind this week's Brussels bombings also spied on a top nuclear researcher and hoped to build a so-called "dirty bomb," an expert involved in a probe into ISIS threats told NBC News on Thursday.

Khalid and Ibrahim El Bakraoui were responsible for planting a hidden camera outside the Belgian researcher's house, according to Claude Moniquet, a French former intelligence official who was hired to investigate potential plots targeting Europe's nuclear sector.

This camera produced more than 10 hours of film showing the comings and goings of senior researcher at a Belgian nuclear center and his family — footage that was seized during a Belgian raid in November, officials announced last month.

Moniquet, who is CEO of the European Strategic Intelligence and Security Center private consultancy, revealed to NBC News on Thursday that the El Bakraoui brothers were behind that failed plot.

"We can imagine that the terrorists might want to kidnap someone or kidnap his family"

"The terrorist cell ... naively believed they could use him to penetrate a lab to obtain nuclear material to make a dirty bomb," he said.

While the plot "did not happen" the investigation confirmed that it was the el Bakraoui brothers who "put a camera in front of the house to take the images."

Belgium's Federal Agency for Nuclear Control confirmed to NBC News on Thursday that the nuclear expert who was spied on was "one of the top researchers" at the SCKCEN Belgian nuclear research center in Mol, 45 miles northeast of Brussels.

"If terrorists are filming one of the top guys in the research center then it is very serious," FANC spokesman Lodewijk van Bladel said. "It means they are planning to do something afterwards with that information ... I don't think they going to make a family movie."

Bladel would not comment on the identity of the perpetrators due to the ongoing investigation.