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 \sim 40 M _ { \odot } and effective spin of \chi _ { eff } \sim 0.5 are the typically expected values from hierarchical mergers within the disks of active galactic nuclei .
While we can not rule out an isolated-binary origin , our results are suggestive ( albeit not definitive , with a Bayes factor of \sim 10 ) , especially together with the binary merger GW170729 , that some gravitational-wave observations may come from hierarchical mergers .