  We present a criterion for multiparticle entanglement based on covariance
matrices. On the one hand, the criterion allows to detect bound entangled
states which are not detected by other criteria; on the other hand, some
strongly entangled pure states such as the GHZ states are not detected. We
show, however, that this is a general phenomenon: No separability criterion
based on two-particle correlations can recognize the entanglement in the family
of graph states, to which the GHZ states belong.
