Intense atmospheric bremsstrahlung X-rays of energies >120 keV were detected by a balloon borne X-ray experiment on September 30, 1976, flown from Fairbanks, Alaska. The X-ray energy spectra have been obtained and compared to the primary electron energy spectra responsible for the X-rays. Using the Monte Carlo results, which predict the X-ray fluxes at various atmospheric depths, we deduce that the electron energy spectra responsible for the X-rays have e-folding energies around 200 keV (average energy >300 keV) if it is assumed that the primary electron energy spectra incident on top of the atmosphere is exponential in form. The event reported here falls in the category of relativistic electron precipitation event.