  The gravitational-wave signal GW150914 was first identified on Sept 14 2015
by searches for short-duration gravitational-wave transients. These searches
identify time-correlated transients in multiple detectors with minimal
assumptions aboutthe signal morphology, allowing them to be sensitive to
gravitational waves emitted by a wide range of sources including binary
black-hole mergers. Over the observational period from September 12th to
October 20th 2015, these transient searches were sensitive to binary black-hole
mergers similar to GW150914 to an average distance of $\sim 600$ Mpc. In this
paper, we describe the analyses that first detected GW150914 as well as the
parameter estimation and waveform reconstruction techniques that initially
identified GW150914 as the merger of two black holes. We find that the
reconstructed waveform is consistent with the signal from a binary black-hole
merger with a chirp mass of $\sim 30 \, M_\odot$ and a total mass before merger
of $\sim 70 \, M_\odot$ in the detector frame.
