  Long and short gamma-ray bursts (GRBs), canonically separated at around 2
seconds duration, are associated with different progenitors: the collapse of a
massive star and the merger of two compact objects, respectively. GRB 191019A
was a long GRB ($T_{90}\sim64$ s). Despite the relatively small redshift
z=0.248 and HST followup observations, an accompanying supernova was not
detected. In addition, the host galaxy did not have significant star formation
activity. Here we propose that GRB 191019A was produced by a binary compact
merger, whose prompt emission was stretched in time by the interaction with a
dense external medium. This would be expected if the burst progenitor was
located in the disk of an active galactic nucleus, as supported by the burst
localization close to the center of its host galaxy. We show that the light
curve of GRB 191019A can be well modeled by a burst of intrinsic duration t=1.1
s and of energy $E_{\rm{iso}}=10^{51}$ erg seen moderately off-axis, exploding
in a medium of density $10^7-10^8$ cm$^{-3}$. The double-peaked light curve
carries the telltale features predicted for GRBs in high-density media, where
the first peak is produced by the photosphere, and the second by the overlap of
reverse shocks that take place before the internal shocks could happen. This
would make GRB 191019A the first confirmed stellar explosion from within an
accretion disk, with important implications for the formation and evolution of
stars in accretion flows and for gravitational waves source populations.
