  In LIGO's O1 and O2 observational runs, the detectors were sensitive to
stellar mass binary black hole coalescences with component masses up to
$100\,M_\odot$, with binaries with primary masses above $40\,M_\odot$
representing $\gtrsim90\%$ of the total accessible sensitive volume.
Nonetheless, of the 5.9 detections (GW150914, LVT151012, GW151226, GW170104,
GW170608, GW170814) reported by LIGO-Virgo, the most massive binary detected
was GW150914 with a primary component mass of $\sim36\,M_\odot$, far below the
detection mass limit. Furthermore, there are theoretical arguments in favor of
an upper mass gap, predicting an absence of black holes in the mass range
$50\lesssim M\lesssim135\,M_\odot$. We argue that the absence of detected
binary systems with component masses heavier than $\sim40\,M_\odot$ may be
preliminary evidence for this upper mass gap. By allowing for the presence of a
mass gap, we find weaker constraints on the shape of the underlying mass
distribution of binary black holes. We fit a power-law distribution with an
upper mass cutoff to real and simulated BBH mass measurements, finding that the
first 3.9 BBHs favor shallow power law slopes $\alpha \lesssim 3$ and an upper
mass cutoff $M_\mathrm{max} \sim 40\,M_\odot$. This inferred distribution is
entirely consistent with the two recently reported detections, GW170608 and
GW170814. We show that with $\sim10$ additional LIGO-Virgo BBH detections,
fitting the BH mass distribution will provide strong evidence for an upper mass
gap if one exists.
