  The origins of the coalescing binary black holes (BBHs) detected by the
advanced LIGO/Virgo are still in debate and clues may present in the mass and
effective spin ($\chi_{\rm eff}$) distributions of these merger events. Here we
analyze the GWTC-1 and GWTC-2.1 BBH events, and find evidence for a primary
mass dependent $\chi_{\rm eff}$ distribution. Below (above) the mass of $\sim
50M_\odot$, a nearly zero (relatively high) effective spin is typical. In the
joint analysis of the primary mass and $\chi_{\rm eff}$ distributions, we
identify a sub-population of BBHs characterized by both a bump-like primary
mass distribution extending to $\sim 100M_\odot$ and a relatively high
$\chi_{\rm eff}\sim 0.3$, consistent with the expectation for the $\geq 2$g
merger events in the disks of active galactic nuclei (AGNs). Our results
suggest that the merger rate in AGN disks is around $1.7~{\rm Gpc}^{-3}{\rm
yr}^{-1}$.
