The Brussels terrorists may have been plotting to make a radioactive bomb after it emerged 11 nuclear power plant workers have had their site access revoked amid fears of "insider help".
Brothers Khalid and Ibrahim el-Bakraoui had secretly filmed the daily routine of Belgium's nuclear programme chief, before they blew themselves up at Brussels airport and on a Metro train.
It is thought the brothers' spying operation was possible preparation for a kidnap plot to force him to let them into one of Belgium's two atomic facilities.
However, it is possible they switched targets to the less well-guarded airport and Maelbeek Metro station after authorities became suspicious.
It emerged yesterday that 11 workers at the Tihange nuclear power plant, 65km southeast of Brussels, have had their access badges withdrawn - leading to speculation the bombers may have been seeking insider help.
Seven of the passes were withdrawn last week, and last Friday additional soldiers were deployed to protect the plant, which has three reactors.
If the terrorists had struck there, it could have caused a catastrophe. The plant lies in a heavily populated area about 80km from each of Belgium's borders with Germany, France, the Netherlands and Luxembourg.
Alternatively, the brothers could have been plotting to steal nuclear material to make a dirty bomb.
Claude Moniquet, who investigates threats to Europe's nuclear sector, told NBC News: "The terrorist cell naively believed they could use him to penetrate a lab to obtain nuclear material to make a dirty bomb."
Belgium's La Derniere Heure newspaper quoted a police source as saying the Bakraoui brothers had hidden cameras in bushes outside the home of the head of the country's nuclear research and development programme.
A Belgian prosecutor refused to divulge the individual's identity "for obvious security reasons".
The footage was found in December during a police raid in Brussels linked to the Paris attacks a month earlier.
The filming confounded investigators at first because it showed the entrance to the director's home in Flanders, an area outside the capital.
But after watching all 12 hours' worth of footage, detectives drew the chilling conclusion that the men were trying to gain access to an atomic facility.
Armed troops were sent to defend French and Belgian nuclear facilities following the discovery and both countries' nuclear programmes were put on the highest state of alert.
On February 17, Belgian prosecutors confirmed the existence of the video seized in December. At the time, investigators knew the video camera had been removed by two men, who left the area in a vehicle with the lights off, but did not know their identity.
La Derniere Heure said it was now clear it was the two brothers.