PETROZAVODSK, Russia -- Historian Yury Dmitriyev has a morbid talent for finding mass graves moldering beneath the forests of the Russia's northern Karelia region.

"I remember he told me once that when there is a burial site, as the organic material turns to dust, the earth settles around the spot," says Yury Brodsky, a historian who studies the Solovki GULAG labor camp. "And he told me how in the spots where these burials were, although they aren't visible, in the early morning you can notice how the dew there evaporates quickly and how in the depressions you can see what he called 'weeping grass.'"

For half of his life, 61-year-old Dmitriyev has worked to tease out tragic secrets of the Stalinist era in his region. He painstakingly assembled a Book of Remembrance that includes the names of 13,000 Great Terror victims.

But in December, he was arrested in his apartment. He now faces charges of "preparing and circulating child pornography." His trial could begin as early as next month, and, if convicted, he faces between eight and 15 years in prison. He also faces an illegal-weapons charge stemming from a piece of a vintage hunting rifle that investigators found in his flat -- a relic his lawyer says the historian recovered during a research dig.

Dmitriyev denies all the allegations, and colleagues both in Russia and abroad argue that the case has been trumped up to punish him for his past research and to silence him in the future. He has been held in remand prison since his arrest.

'People, Don't Kill Another!'

In 1997, Dmitriyev, who is the head of Karelia's chapter of the rights-advocacy group Memorial, and his colleagues discovered the Sandarmokh mass grave, where more than 9,500 prisoners representing over 60 ethnic groups from the Solovki GULAG camp were executed and buried by Stalin's secret police in 1937-38.

In addition, over strong resistance from Karelian security agents, Dmitriyev lobbied tirelessly to have the spot memorialized. As a result, today there is a large stone memorial at the site bearing the words that Dmitriyev chose to honor the spot: "People, don't kill one another!"

Most recently, he participated in a Memorial project to create an online database giving details about some 40,000 agents of Stalin's secret police, who were directly responsible for the murders of some 1 million Soviet citizens during the Great Terror period, as well as the persecution of millions more

"Dmitriyev got phone calls about the 'executioners' database,'" says Irina Korneichuk, who has known Dmitriyev for years and has worked with him on expeditions to Solovki, in the far north, and Lodeinoye Polye, near St. Petersburg. "A lot of them."

Russia this year marks the 80th anniversary of 1937, the deadliest year of Stalin's murderous rampage.

Images Of Foster Daughter

Prosecutors say the case against Dmitriyev began with an anonymous denunciation. Investigating the tip, authorities found a folder on his computer containing 49 nude photographs of his foster daughter, now 11, that were taken between 2012 and 2015. The folder was labeled "Child's Health."

Dmitriyev and his wife took in the girl, Natasha, when she was 3 with the intention of adopting her. She was malnourished and in poor health. Dmitriyev's attorney, Viktor Anufriyev, says medical workers asked Dmitriyev to monitor her health and development. Each photograph was accompanied by notation of the girl's height, weight, and general health. Many of them appear to have been taken ahead of scheduled visits by social workers, possibly to document that Natasha had been well treated. According to social-services reports, nothing suspicious was ever noted.

Memorial activist Sergei Krivenko tells RFE/RL that Dmitriyev successfully completed a state training course aimed at would-be adoptive parents.

"During those courses, he was told that it was essential to closely monitor the child's development and to record everything in notebooks -- to record the state of her development and so on," Krivenko says. "And this fit exactly into his character as a researcher. He always took meticulous care with things that had to be recorded accurately. That is why he decided to keep this photographic record of his daughter's development."

Fellow researcher Brodsky makes a similar observation. "In general, he photographed everything, particularly during a dig," he tells RFE/RL. "Life taught him this habit. It was his approach, his methodology. He photographed everything and he wrote everything down. That's just the way he thinks. And he did everything so Natasha would flourish and develop physically. This wasn't a normal, healthy child -- but precisely a neglected child that he put on the right path."

Rallying Support

Dmitriyev's defense team has not been allowed to see the anonymous letter and is at a loss to explain it because the historian insists he never showed the photographs to anyone. But shortly before his arrest, Dmitriyev was unexpectedly summoned to the police station. When he returned home several hours later, he says he discovered that someone had been using his computer.

Memorial colleagues suspect the authorities were looking for information about his latest research or about the "executioners' database" project and may have stumbled on Natasha's records accidentally.

"I lived in Dmitriyev's home and saw him relating with his foster daughter, Natasha," says Irina Korneichuk, who worked with the historian on several expeditions. "They were related souls, saw things eye to eye and understood each other immediately. It was exactly what many people are missing in their lives. Maybe he was more of a grandfather to her than a father."

"She traveled with him on his expeditions," Korneichuk continues. "She was on all his trips. Listened to those grown-up discussions, worked with the kids from the film schools [who were documenting the expeditions]. She had an interesting, colorful life full of understanding and concern."

After Dmitriyev's arrest, the film students with whom he'd worked in recent years opened a fund to pay for his legal defense. More than 7,000 people have signed an online petition denouncing the charges against the historian as "a provocation." Historians and activists from Ukraine and Poland have published open letters of support.

"We are certain that the criminal case against Dmitriyev is the persecution of a historian and archivist for his public activity," Memorial's Krivenko says. "The accusations against him have nothing in common with everything that is known about Dmitriyev."

During the investigation and while Dmitriyev was in custody, the historian's latest research -- 450 pages of meticulous investigation -- and his entire research archive disappeared without a trace.

Robert Coalson contributed to this report