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We have carried out a systematic analysis of the gamma-ray bursts' (GRBs)light curves detected in the SPI-ACS experiment onboard the INTEGRALobservatory aimed to search extended emission. The emission occasionallyrecorded after the prompt active phase of a GRB in the form of an emission thatis longer than the active phase and less intense is called the extended one.Out of the 739 brightest GRBs recorded from 2002 to 2017, extended emission hasbeen detected in $\sim20\%$ of the individual light curves; its maximumduration reaches $\sim 10000$ s. Two different types of extended emission havebeen revealed. One of them is an additional component of the light curve and isdescribed by a power law (PL) with an index $\alpha \sim -1$ close to the PLindex of the afterglow in the optical and X-ray bands. The second type can bedescribed by a steeper PL decay of the light curve typical of the active burstphase. Extended emission has also been found in the combined light curve oflong GRBs in the individual curves of which no extended emission has beendetected. The PL index of the extended emission in the combined light curve is$\alpha \sim -2.4$. It is most likely associated with the superposition oflight curves at the active phase; its total duration is $\sim 800$ s.

Extended Emission of Cosmic Gamma-Ray Bursts Detected in the SPI-ACS/INTEGRAL Experiment