We present the spectrum of stimulated THz emission of uniaxially stressed p-Ge resonant-state laser (RSL) measured at low electric fields at which the continuous-wave operation of RSL was observed. The spectral lines observed at low voltages are due to intracenter optical transitions. The line energies show that the low-voltage lasing is due to an inverted population of resonant states of acceptors induced by strain. The population inversion of the resonant states exists both at low fields when hole heating is diffusive and at high fields when the hole heating is ballistic. A new line caused by optical transitions between continuum and resonant states is found.