  The financial crisis clearly illustrated the importance of characterizing the
level of 'systemic' risk associated with an entire credit network, rather than
with single institutions. However, the interplay between financial distress and
topological changes is still poorly understood. Here we analyze the quarterly
interbank exposures among Dutch banks over the period 1998-2008, ending with
the crisis. After controlling for the link density, many topological properties
display an abrupt change in 2008, providing a clear - but unpredictable -
signature of the crisis. By contrast, if the heterogeneity of banks'
connectivity is controlled for, the same properties show a gradual transition
to the crisis, starting in 2005 and preceded by an even earlier period during
which anomalous debt loops could have led to the underestimation of
counter-party risk. These early-warning signals are undetectable if the network
is reconstructed from partial bank-specific data, as routinely done. We discuss
important implications for bank regulatory policies.

Physics and Society;Data Analysis, Statistics and Probability;General Finance;Risk Management