Complex sentence: In the first observational run of Advanced LIGO detectors, from September 12,2015 to January 19,2016, it also indentified a third possible signal but is not significant enough (about 2σ) to become a gravitational wave event.
Split into simpler sentences: In the first observational run of Advanced LIGO detectors, from 12 September 2015 to 19 January 2016, they identified a third possible signal. This observation was not significant enough (about 2σ) to be declared a gravitational wave event.