A Russian court extended the 13-year prison term Monday for a Russian historian of Josef Stalin-era repression by an additional two years, making his sentence 15 years.

Yuri Dmitriyev, who uncovered mass graves of Stalinist victims, was found guilty last year of sexually abusing his adopted daughter. Supporters of Dmitriyev claimed the ruling was politically motivated and said the charge was fabricated. Dmitriyev also denied the charges.

Dmitriyev, 65, was also charged with indecent acts, illegal possession of a part of a weapon, and creating child pornography after allegedly taking naked photos of his daughter. However, experts initially said the photographs were not pornography.

Dmitriyev was acquitted of all charges in 2018. However, the case was reopened a few months later. He was found guilty of sexual assault in July of 2020 and sentenced to 3 1/2 years in prison, yet a few months later, the sentence was extended by a decade by the Russian Supreme Court.

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The Petrozavodsk City Court, near the border of Finland, extended the sentence again Monday by adding two years. Dmitriyev's attorneys said they plan to appeal the ruling, according to the Associated Press.

The Human Rights Center Memorial, where Dmitriyev used to be the head of the branch in the state of Karelia, recognizes Dmitriyev as a political prisoner. Other prominent figures and unions have called upon Russia to drop the charges, including the European Union.

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The United States Embassy in Russia last year condemned the 13-year sentence by the Russian Supreme Court.

“The Karelian supreme court’s decision to prolong historian Yury Dmitriev’s already unjust sentence by an outrageous 10 additional years is another step backwards for human rights and historical truths in Russia,” spokeswoman Rebecca Ross said, according to the Guardian.