MOSCOW — A Russian historian of the Soviet forced labor camps known as the Gulag, whose work ran counter to official contemporary narratives that play down the crimes of the Stalin era, was acquitted on Thursday of child pornography charges, which human rights groups said had been trumped up for political reasons.

The historian, Yuri A. Dmitriev, gained renown in 1997 for discovering the remains of more than 9,000 victims of Stalinist purges buried in communal pits in Sandarmokh, a forest in northwestern Russia.

Mr. Dmitriev has devoted the past 30 years to searching for such mass graves together with colleagues from Memorial, one of Russia’s oldest and most respected human rights organizations. Mr. Dmitriev serves as the regional director of Memorial in Karelia, a region in northwestern Russia bordering Finland.

Initially, local government officials had supported his efforts and attended memorial events at the forest, which has been commemorated with a chapel and monuments to the victims amid the trees.