  We present the results of the optical spectroscopy follow-up of a sample of
28 unclassified blazars from the Third Fermi-LAT Catalog of High-Energy Sources
(3FHL). All the spectra were taken with the 4m Mayall telescope at Kitt Peak.
With this follow-up program we are able to classify 27 out of 28 objects as BL
Lacs, while the remaining one is a flat spectrum radio quasar. We determine a
redshift (z) for three of these objects and a lower limit on z for other four
sources: the farthest object for which we obtain a redshift has z>0.836. These
results are part of a more extended campaign of optical spectroscopy follow-up
of 3FHL blazars, aimed to obtain a complete sample of blazars at >10 GeV which
will then be used to extend our knowledge on blazar emission mechanisms and on
the extragalactic background light.
