Brazilian prosecutors have opened an investigation into three penitentiaries in Goias state after videos surfaced showing what seem to be prison guards torturing inmates. In videos believed to have been shot between 2014 and 2015, but only obtained this week by prosecutors, several guards can be seen repeatedly using stun guns on inmates who were already under control. Violence is rife in Brazil's overcrowded prison system. Earlier this year several prison riots left dozens of inmates dead.