On 14 September 2015, a gravitational wave signal from a coalescing blackhole binary system was observed by the Advanced LIGO detectors. This paperdescribes the transient noise backgrounds used to determine the significance ofthe event (designated GW150914) and presents the results of investigationsinto potential correlated or uncorrelated sources of transient noise in thedetectors around the time of the event. The detectors were operating nominallyat the time of GW150914. We have ruled out environmental influences andnon-Gaussian instrument noise at either LIGO detector as the cause of theobserved gravitational wave signal.