A construction of covers of arithmetic schemes

Let X be a regular arithmetic scheme, i.e. a regular integral separated scheme flat and of finite type over SpecZ. Assume that for all closed irreducible subschemes C⊆X of dimension 1 with normalisation there are given open normal subgroups NC of , which fulfil the following compatibility condition: For all the pre-images of NC1 and NC2 in coincide. If the indices of the NC are bounded, then these data uniquely determine an open normal subgroup of π1(X), whose pre-image in is NC for all C.