  The $Z^\pm_c(3900)/Z^\pm_c(3885)$ resonant structure has been experimentally
observed in the $Y(4260) \to J/\psi \pi\pi$ and $Y(4260) \to \bar{D}^\ast D
\pi$ decays. This structure is intriguing since it is a prominent candidate of
an exotic hadron. Yet, its nature is unclear so far. In this work, we
simultaneously describe the $\bar{D}^\ast D$ and $J/\psi \pi$ invariant mass
distributions in which the $Z_c$ peak is seen using amplitudes with exact
unitarity. Two different scenarios are statistically acceptable, where the
origin of the $Z_c$ state is different. They correspond to using energy
dependent or independent $\bar D^* D$ $S$-wave interaction. In the first one,
the $Z_c$ peak is due to a resonance with a mass around the $D\bar D^*$
threshold. In the second one, the $Z_c$ peak is produced by a virtual state
which must have a hadronic molecular nature. In both cases the two
observations, $Z^\pm_c(3900)$ and $Z^\pm_c(3885)$, are shown to have the same
common origin, and a $\bar D^* D$ bound state solution is not allowed. Precise
measurements of the line shapes around the $D\bar D^*$ threshold are called for
in order to understand the nature of this state.
