In this letter, we present for the first time simultaneously measured electron and gamma ray spectra of TGE. We also demonstrate how the measurement of the energy spectra of TGE particles can help to understand the charge structure of the thundercloud. We introduce two main scenarios that can support the initiation of the relativistic runaway avalanches (RREA) in the thundercloud. One of the scenarios includes the emergence of the lower positive charged layer (LPCR). The LPCR is a short-living charge structure sitting on hydrometeors, which exhibit accidental charge reversals depending on temperature changes. LPCR can sustain a positive (downwards directed) near-surface electric field for several minutes, before falling down with precipitation. Comparison of energy spectra of electrons and gamma rays makes it possible to scrutinize the LPCR emergence and estimate the height where the strong electric field that originates the RREA declines.