We construct examples of smooth 4-dimensional manifolds M supporting a
locally CAT(0)-metric, whose universal cover X satisfy Hruska's isolated flats
condition, and contain 2-dimensional flats F with the property that the
boundary at infinity of F defines a nontrivial knot in the boundary at infinity
of X. As a consequence, we obtain that the fundamental group of M cannot be
isomorphic to the fundamental group of any Riemannian manifold of nonpositive
sectional curvature.