A human rights group in Russia says Yury Dmitriyev, the imprisoned historian and former head of the Memorial human rights group in the northwestern region of Karelia, is being mistreated at his prison in Mordovia.

The Memorial Society said on September 28 that Dmitriyev had been placed in punitive solitary confinement three times since mid-September for unwarranted reasons.

According to Memorial, Dmitriyev was initially sent to solitary confinement, a tiny concrete room with no toilet or running water, for three days on September 16 for failing to properly greet a guard. After serving that punishment, Dmitriyev was immediately returned to the punitive cell for five days for having a cat on his bed.

The human rights group added that on September 26, the administration of the prison in Mordovia -- an area historically associated with some of Russia's most brutal prisons, including Soviet-era labor camps for political prisoners -- again put the 66-year-old historian in solitary confinement for five days for "failing to quickly follow a guard’s command to put his hands behind his back."

"Constant and baseless placement in a punitive cell is one of the known methods of pressure imposed by penitentiary administrations on inmates," Memorial said, adding that it continues to follow the historian's time in the prison.

The high-profile case against Dmitriyev dates back to 2016, when the academic, who spent decades researching extrajudicial executions carried out in Karelia under Soviet dictator Josef Stalin, was arrested over photographs of his foster daughter that investigators found on his computer.

The authorities said the images were pornographic, but Dmitriyev said they were made at the request of social workers concerned about the child's physical development.

He was acquitted in April 2018, but the Karelia Supreme Court upheld an appeal by prosecutors and ordered a new trial. He was rearrested in June 2018 and then charged with the more serious crime of sexual assault against a minor.

In July 2020, Dmitriyev was sentenced to 3 1/2 years for "violent acts of a sexual nature committed against a person under 14 years of age." He has rejected the charge, insisting that he is being targeted because of his research into the crimes of Stalin's regime.

Prosecutors, who had asked for 15 years in prison in the high-profile case, said the original sentence was "too lenient" and appealed it. Dmitriyev's defense team, meanwhile, also appealed, insisting he was innocent.

In September 2020, weeks before he was due to be released because of time served, the Supreme Court of Karelia accepted the prosecutors' appeal and added another 9 1/2 years onto Dmitriyev's sentence.

Dozens of Russian and international scholars, historians, writers, poets, and others have issued statements in support of the scholar, while the European Union has called for Dmitriyev to be released.

Dmitriyev's research has been viewed with hostility by the government of President Vladimir Putin. Under Putin, Stalin has undergone a gradual rehabilitation, and the Russian government has emphasized his leadership of the Soviet Union while downplaying his crimes against Soviet citizens.

Under Stalin, millions of people were executed, sent to labor camps, or starved to death in famines caused by forced collectivization. During World War II, entire ethnic groups were deported to remote areas as collective punishment for alleged collaboration with the Nazis.