  The OPERA experiment has reported the detection of five $\nu_{\tau}$
candidates in the CNGS $\nu_{\mu}$ beam, allowing to reject the background-only
hypothesis at the 5.1$\sigma$ level. Besides these events, on May 23$^{rd}$
2011, OPERA detected a "neutral current like" interaction with two secondary
vertices. Such topologies mainly arise from Charged Current interactions of a
$\nu_{\tau}$ with associated charm quark production or from Neutral Current
interactions of a $\nu_{\mu}$ with production of a charm anti-charm pair. These
topologies have generally low probabilities. A dedicated multivariate analysis
is in progress to allow discriminating between these two hypotheses. Here the
event topology is described in detail and preliminary results of the
classifiers for all possible contributions are given.
