Russia jailed a Stalin-era historian for 13 years on Tuesday after overturning an earlier sentence on charges his supporters allege are made up to punish him for his documentations.

Yuri Dmitriev, a gulag researcher and human rights activist, was found guilty in July of sexually abusing his adopted daughter and was sentenced to three and a half years in prison by the Petrozavodsk City Court in Russia's northwestern Karelia region, according to Reuters.

Dmitriev, 64, denied the charges, which his lawyer and supporters said were fabricated due to his work with the rights group Memorial.

Dmitriev is internationally recognized for his documentations of Soviet dictator Josef Stalin's 1937-38 Great Terror. The era, also called the Great Purge, describes the political campaign set out to eliminate those dissenting to the Communist Party or those seen as a threat. Historians say at least 750,000 people were executed, while more than 1 million others were sent to forced labor camps, known as gulags.

Dmitriev was cleared of initial child pornography charges regarding his adopted daughter during a 2018 trial but was subsequently arrested on another charge. The ruling of the first trial was annulled.

In July, Memorial called for the immediate release of Dmitriev, referring to him as a political prisoner.

"This internationally recognized work of Yuri Dmitriev had inflicted misery on him," the group said. "Nowadays, when the political repressions are becoming frequent in our country again, and the truth about the past does not fit into the statist concept of history, an authorities’ hostile attitude towards the preservation of the memory of state political terror is predictable."

The initial July sentence would have freed Dmitriev by November thanks to time served, but Karelia's Supreme Court said he would now be held for 13 years in a high-security penal colony. A lawyer for Dmitriev said he would appeal the decision.

Rebecca Ross, spokeswoman for the U.S. Embassy in Moscow, condemned the ruling in a tweet on Tuesday.

"The Karelian Supreme Court’s decision to prolong historian Yuri Dmitriyev’s already unjust sentence by an outrageous 10 additional years is another step backwards for #humanrights and historical truths in #Russia," Ross said.

The Kremlin has said it is not involved in the case, but state prosecutors have said they believe the case is based on real evidence and is not politically motivated.