A Russian court on Wednesday sentenced respected Gulag historian Yury Dmitriyev to more than three years in prison in a controversial sexual abuse case, his lawyer said.

Dmitriyev, head of the prominent rights group Memorial in Karelia in northwestern Russia, spent decades locating and exhuming mass graves of people killed under Soviet dictator Joseph Stalin’s rule.

Memorial said the prosecution of the researcher, who was tried behind closed doors, is part of a growing crackdown on dissenters.