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It is generally assumed that for energy injection before recombination, allof the injected energy is dissipated as heat in the baryon-photon plasma,giving rise to the $y$-type, $i$-type, and $\mu$-type distortions in the CMBspectrum. We show that this assumption is incorrect when the energy is injectedin the form of energetic (i.e. energy much greater than the background CMBtemperature) particles. We evolve the electromagnetic cascades, from theinjection of high energy particles, in the expanding Universe and follow thenon-thermal component of CMB spectral distortions resulting from theinteraction of the electromagnetic shower with the background photons,electrons, and ions. The electromagnetic shower loses a substantial fraction ofits energy to the CMB spectral distortions before the energy of the particlesin the shower has degraded to low enough energies that they can thermalize withthe background plasma. This spectral distortion is the result of theinteraction of non-thermal energetic electrons in the shower with the CMB andthus has a shape that is substantially different from the $y$-type or $i$-typedistortions. The shape of the final \emph{non-thermal relativistic}($ntr$-type) CMB spectral distortion depends upon the initial energy spectrumof the injected electrons, positrons, and photons and thus has informationabout the energy injection mechanism e.g. the decay or annihilation channel ofthe decaying or annihilating dark matter particles. The shape of the spectraldistortion is also sensitive to the redshift of energy injection. Ourcalculations open up a new window into the energy injection at $z\lesssim2\times 10^5$ which is not degenerate with, and can be distinguished from thelow redshift thermal $y$-type distortions.

Rich structure of non-thermal relativistic CMB spectral distortions from high energy particle cascades at redshifts $z\lesssim 2\times 10^5$