  The high statistics COMPASS results on diffractive dissociation $\pi N
\rightarrow \pi \pi \pi N$ suggest that the isospin $I=1$ spin-parity $J^{PC}=
1^{++}$ $a_1(1260)$ resonance could be split into two states: $a_1(1260)$
decaying into an S-wave $\rho\pi$ system, and $a_1^\prime(1420)$ decaying into
a P-wave $f_0(980)\pi$ system. We analyse the reaction by incorporating our
previous treatment of resonant re-scattering corrections in the Drell-Deck
forward production process. Our results show that the COMPASS results are fully
consistent with the existence of a single axial-vector $a_1$ resonance. The
characteristic structure of the production process, which differs in the two
orbital angular momentum states, plays a crucial role in this determination.
Provided the theoretical analysis of the reaction is done in a consistent
manner, this single resonance produces two peaks at different locations in the
two channels, with a rapid increase of the phase difference between their
amplitudes arising mainly from the structure of the production process itself,
and not from a dynamical resonance effect. In addition, this analysis clarifies
questions related to the mass, width, and decay rates of the $a_1$ resonance.
