  We present high S/N optical spectra of 10 BL Lac objects detected at GeV
energies by Fermi satellite (3FGL catalog), for which previous observations
suggested that they are at relatively high redshift. The new observations,
obtained at the 10 m Gran Telescopio Canarias, allowed us to find the redshift
for J0814.5+2943 (z = 0.703) and we can set spectroscopic lower limit for
J0008.0+4713 (z>1.659) and J1107.7+0222 (z>1.0735) on the basis of Mg II
intervening absorption features. In addition we confirm the redshifts for
J0505.5+0416 (z=0.423) and for J1450+5200 (z>2.470). Finally we contradict the
previous z estimates for five objects (J0049.7+0237, J0243.5+7119,
J0802.0+1005, J1109.4+2411, and J2116.1+3339).
