  We make use of a unitarized coupled-channel model to analyze the mass
distribution data of final states in production processes of $X(4260)$. By
analyzing the analytical structures of the decay amplitudes, we find that the
line shape of $Z_c(3900)$ signal is related to the combined effect of a pair of
near-threshold "shadow" poles and the $(D\bar{D}^*)^\pm$ thresholds, in which
the third-sheet pole might provide a dominant contribution. As all the coupled
channels effects are tuning off, the trajectories of these two poles suggest
that the $Z_c(3900)$ might originate from the attractive interaction of
$(D^*\bar{D}^*)^\pm$ through a long-distance interaction, $e.g.$ $\pi$-exchange
interaction, as a "deuteron-like" state. There is no nearby pole structure
corresponding to the $Z_c(4025)$ signal in the $(D^*\bar{D}^*)^\pm$ mass
distribution.
