  The connection between short gamma-ray bursts (SGRBs) and binary neutron star
(BNS) mergers was recently confirmed by the association of GRB 170817A with the
merger event GW170817. However, no conclusive indications were obtained on
whether the merger remnant that powered the SGRB jet was an accreting black
hole (BH) or a long-lived massive neutron star (NS). Here, we explore the
latter case via BNS merger simulations covering up to 250 ms after merger. We
report, for the first time in a full merger simulation, the formation of a
magnetically-driven collimated outflow along the spin axis of the NS remnant.
For the system at hand, the properties of such an outflow are found largely
incompatible with a SGRB jet. With due consideration of the limitations and
caveats of our present investigation, our results favour a BH origin for GRB
170817A and SGRBs in general. Even though this conclusion needs to be confirmed
by exploring a larger variety of physical conditions, we briefly discuss
possible consequences of all SGRB jets being powered by accreting BHs.
