  We describe a stream-based analysis pipeline to detect gravitational waves
from the merger of binary neutron stars, binary black holes, and
neutron-star-black-hole binaries within ~ 1 minute of the arrival of the merger
signal at Earth. Such low-latency detection is crucial for the prompt response
by electromagnetic facilities in order to observe any fading electromagnetic
counterparts that might be produced by mergers involving at least one neutron
star. Even for systems expected not to produce counterparts, low-latency
analysis of the data is useful for deciding when not to point telescopes, and
as feedback to observatory operations. Analysts using this pipeline were the
first to identify GW151226, the second gravitational-wave event ever detected.
The pipeline also operates in an offline mode, in which it incorporates more
refined information about data quality and employs acausal methods that are
inapplicable to the online mode. The pipeline's offline mode was used in the
detection of the first two gravitational-wave events, GW150914 and GW151226, as
well as the identification of a third candidate, LVT151012.
