  Dark matter in the Milky Way may annihilate directly into gamma rays,
producing a monoenergetic spectral line. Therefore, detecting such a signature
would be strong evidence for dark matter annihilation or decay. We search for
spectral lines in the Fermi Large Area Telescope observations of the Milky Way
halo in the energy range 200 MeV to 500 GeV using analysis methods from our
most recent line searches. The main improvements relative to previous works are
our use of 5.8 years of data reprocessed with the Pass 8 event-level analysis
and the additional data resulting from the modified observing strategy designed
to increase exposure of the Galactic center region. We searched in five sky
regions selected to optimize sensitivity to different theoretically-motivated
dark matter scenarios and find no significant detections. In addition to
presenting the results from our search for lines, we also investigate the
previously reported tentative detection of a line at 133 GeV using the new Pass
8 data.
