We study the long-period earthquakes that occur at the crust-mantle boundary beneath the Klyuchevskoy volcano group in Kamchatka in order to reconstruct their source mechanisms. These earthquakes are observed at frequencies between 1 and 4 Hz and the phases of their seismograms are strongly affected by the high-pass filtering required to remove the microseismic noise. Therefore, we decided to use an inversion method based on amplitude ratios between S- and P-waves. Considering the uncertainties associated with this method and potentially leading to non-uniqueness of the inversion, we decided not to explore the full space of the source parameters but to test a set of “elementary” mechanisms corresponding to processes possibly occurring within magmatic systems of volcanoes and their surroundings. Also, after measuring the raw amplitudes of P- and S-waves we corrected them for the site amplification effects. Based on the results of the inversion, the generation of the DLP earthquakes beneath Klyuchevskoy by shear faulting (as would be the case in thermomechanical stresses associated with cooling of deep intrusions) can be reasonably excluded. The observed signal amplitudes can be better explained with source mechanisms containing strong volumetric or single force components. The former can be associated with the pressure perturbation withing magmatic reservoirs or conduits and the latter with the sudden acceleration of the magma movement. The ensemble of our observations is compatible with the configuration when the magma is stored in nearly horizontal sills near the crust-mantle boundary and penetrates into the crust through conduits dipping south-southwest, in agreement with previously reported connection of the deep magmatic reservoir with the Bezymanny and Tolbachik volcanoes.