Yuri Dmitriyev had just been handed a three-month non-custodial sentence on flimsy charges, though you might not have guessed it from the scenes of jubilation as he exited the courtroom, embraced by friends and family.

But for the historian and anti-Gulag activist there was plenty to celebrate, having been acquitted of far more damaging charges of child pornography. He had stood accused of producing indecent images of his 11-year-old adopted daughter. All along, he and his legal team argued that the trial was an act of vengeance by the authorities.

“It’s not exactly right to accept congratulations. I knew I wasn’t a paedophile,” said Dmitriyev immediately after the verdict.

Dmitriyev’s 15-month trial was contested in the shadow of Russia’s past and its all-powerful security services. For supporters, it lurched from hope to despair. In December 2017, the judge ruled not to extend his arrest – and that was seen as the beginning of the end. But in January that news was superseded by a nine-year prison sentence request from state prosecutors. Given the near 100 per cent conviction rate in Russian courtrooms, few expected a good outcome.

In the event, the judge’s decision to pass a two-and-a-half-year sentence for a lesser (and also disputed) firearms possession charge seemed miraculous. The sentence was reduced to three months on account of time already spent in custody. Under Russian law, those three months could be spent at home under restrictions as determined by the judge.

Dmitriyev’s ordeal began in December 2016, when the historian was arrested at his home in Petrozavodsk, following an anonymous tip-off. Police confiscated computers, where they found naked images of his adoptive daughter, contained in a file marked “health diary”. In court, Dmitriyev has insisted there were legitimate reasons for taking the pictures. He had done so on advice, he said, to protect against allegations of battery and child abuse.

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Experts called by the judge agreed with him, ruling the photographs to be non-sexual in content. But as the trial reached its final weeks, the court ordered a second review of the photographs, and for Dmitriyev to undergo forced psychological tests. The historian’s lawyer, Victor Anufriyev, suggested that state prosecutors might be trying to pressure doctors to declare him insane.

There was a sense, said the lawyer, that parts of officialdom held a grudge against Dmitriyev for his efforts uncovering mass, Stalin-era graves in the Russian far north.

During the 1930s, the lands around Dmitriyev’s native Karelia district were an epicentre of the Gulag network and the Great Terror. Sixty years later, using newly declassified police execution lists, Dmitriyev and a team of activists began to search for the victims. The contemporary notes led them to unusual clearings in the forest. They began digging and found several mass burial sites.

In total, Dmitriyev documented more than 13,000 victims, complete with their skulls and bones. Many more victims are believed to remain underground, preserved in the permafrost.

Local authorities initially embraced Dmitriyev’s work, and even funded annual memorial events. But in recent years, they withdrew support. The rethink was consistent with a broader national reassessment of the Stalin era, which has moved away from outright condemnation during Dmitry Medvedev’s brief presidency.

The message from the Kremlin today is contradictory. On the one hand, President Vladimir Putin talked about “unjustifiable” crimes of the past when opening a new memorial wall in Moscow last year. On another, the former KGB agent has cautioned against the “excessive demonization of Stalin”, which he says is “used to undermine Russia”.

Coming so soon in the president’s new term, many speculated that the verdict would lay a marker of the general (and presumably ever-murkier) direction of Russian politics. In its early activity, after all, the Kremlin had shown little sign of backing away from the hard line.