  An event topology with two secondary vertices compatible with the decay of
short-lived particles was found in the analysis of neutrino interactions in the
Opera target. The observed topology is compatible with tau neutrino charged
current (CC) interactions with charm production and neutrino neutral current
(NC) interactions with $c\overline{c}$ pair production. However, other
processes can mimic this topology. A dedicated analysis was implemented to
identify the underlying process. A Monte Carlo simulation was developed and
complementary procedures were introduced in the kinematic reconstruction. A
multivariate analysis technique was used to achieve an optimal separation of
signal from background. Most likely, this event is a $\nu_{\tau}$ CC
interaction with charm production, the tau and charm particle decaying into 1
prong and 2 prongs, respectively. The significance of this observation is
evaluated.
