What type of torture methods did the interrogators at the gulags used to get prisoners to admit to crimes they never commited

The gulag was a Russian government agency created by Vladimir Lenin, the agency was in charge of the soviet forced labor camp system. The camp harbored a vast number of convicts. Some of the methods that interrogators used to get prisoners to admit to crimes they never committed included having prisoners' skulls squeezed within iron rings, exposing their naked bodies to be bitten by ants and bugs and being kept from sleeping for a week.