That’s the question echoing across Western Europe right now, after the German media claimed that one of the prime suspects arrested for the attacks, which rocked the French capital late last year, was also found in possession of documents concerning a nuclear research site close to the Belgium-Germany border.
According to Redaktions Netzwerk Deutschland media group, when officers swooped in on the apartment of Salah Abdeslam - arrested on 18 March in the Brussels district of Molenbeek and currently awaiting a trial for the attacks which killed 130 people - they found internet printouts of the Juelich Centre and also photos of its chairman, Wolfgang Marquardt.
The source behind the leak is believed to be inside a parliamentary control committee, which met with Germany’s domestic intelligence agency chief Hans-Georg Maaßen towards the end of last month, when it was given the news.
News which probably won't be welcomed in some quarters. Not least anyone working for the intelligence agencies tasked with stopping the terror cells in mainland Europe - particularly after the arrest of Abdeslam last month reportedly triggered the follow-up attacks on Brussels airport and the city’s metro system, killing 32 people last month.
Even before Paris there had been reports that Islamic terrorists were looking to exploit weak spots in the nuclear industry, and should this latest threat be genuine, it will only raise further concern over how far they’ll go.