  Despite the rapidly growing number of stellar-mass binary black hole mergers
discovered through gravitational waves, the origin of these binaries is still
not known. In galactic centers, black holes can be brought to each others'
proximity by dynamical processes, resulting in mergers. It is also possible
that black holes formed in previous mergers encounter new black holes,
resulting in so-called hierarchical mergers. Hierarchical events carry
signatures such as higher-than usual black hole mass and spin. Here we show
that the recently reported gravitational-wave candidate, GW170817A, could be
the result of such a hierarchical merger. In particular, its chirp mass
$\sim40$ M$_\odot$ and effective spin of $\chi_{\rm eff}\sim0.5$ are the
typically expected values from hierarchical mergers within the disks of active
galactic nuclei. We find that the reconstructed parameters of GW170817A
strongly favor a hierarchical merger origin over having been produced by an
isolated binary origin (with an Odds ratio of $>10^3$, after accounting for
differences between the expected rates of hierarchical versus isolated mergers)
