We construct examples of 4-dimensional manifolds M supporting a locally CAT(0)-metric, whose universal covers M satisfy Hruska's isolated flats condition, and con-tain 2-dimensional flats F with the property that θ~∞ F ≌ S~1 → S~3 ≌θ~∞ M are nontrivial knots. As a consequence, we obtain that the group π_1(M) cannot be isomorphic to the fundamental group of any compact Riemannian manifold of nonpositive sectional curvature. In particular, if K is any compact locally CAT(0)-manifold, then M × K is a locally CAT(0)-manifold which does not support any Riemannian metric of nonpositive sectional curvature.
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