Last week, the administrative office of the St. Petersburg city parliament called Boris Vishnevsky, one of its few opposition lawmakers, to say a letter had arrived for him from the Foreign Ministry in Moscow. He was told he would not be allowed to keep it; he could only read it at the office and return it straightaway. The letter was marked “for official use,” and Vishnevsky was warned not to publish or send it anywhere. “Fine, I am not sending it anywhere,” he responded. “But I am reporting its contents.”

The letter was a response to Vishnevsky’s inquiry to Foreign Minister Sergei Lavrov about media reports that Emanuelis Zingeris, the Parliamentary Assembly of the Council of Europe’s special rapporteur on the investigation into the murder of Russian opposition leader Boris Nemtsov, had been banned from entering Russia. The letter that Vishnevsky was allowed to see — but not keep — contained official confirmation from the Russian Foreign Ministry that it had indeed issued a travel ban against Zingeris, and that he would not be allowed to enter the country.

It would be difficult to think of a clearer message from the Russian authorities. The travel ban on the Council of Europe’s rapporteur caps years of Kremlin efforts to hinder any meaningful investigation into the most high-profile political assassination in modern Russia. More than three years after Nemtsov, the former deputy prime minister and Vladimir Putin’s most prominent opponent, was gunned down in sight of the Kremlin’s walls, none of the organizers or masterminds of his murder have been identified or brought to justice.

Despite evident links between convicted gunman Zaur Dadayev, who was previously an Interior Ministry officer, and the Kremlin-appointed ruler of Chechnya, Ramzan Kadyrov — who has publicly referred to Dadayev as “a real patriot” and to Nemtsov as “an enemy of Russia” — the Chechen leader was not even once questioned by investigators. Neither was General Viktor Zolotov, a top military commander and one of Putin’s closest confidants. Attempts by investigators to indict another Kadyrov associate, Interior Ministry officer Ruslan Geremeyev, as an organizer in the assassination were personally vetoed by Gen. Alexander Bastrykin, the head of Russia’s Investigative Committee. In one of the most Orwellian episodes of the entire proceedings, the Federal Protective Service refused to hand over video footage from nearby security cameras on the night of the murder, stating that the bridge where Nemtsov was shot dead — two hundred yards from the Kremlin — “is not a protected site.”

Perhaps the most glaring omission involves motive. The Russian authorities deliberately classified the assassination of Nemtsov — a former government minister and regional governor, a four-term member of Russia’s parliament and, at the time of the murder, a regional lawmaker and leader of a political party — under Criminal Code Article 105, on ordinary murder, instead of Article 277, which concerns an attack on “a statesman or public figure.” As state prosecutor Viktor Antipov told the court, “we cannot allow for the murders of opposition members to be handled under Article 277.” Throughout the trial, questions relating to political motives were repeatedly disallowed by the judge. “We saw a trial where it is forbidden to talk about substance,” wrote opposition lawmaker Lev Shlosberg. “The court is investigating a political assassination without any questions about politics.”

And so the Russian authorities have declared the Nemtsov case “solved” — though there has been no questioning of relevant suspects, no identification of motive, no investigation of the organizers or masterminds behind the killing. What the Kremlin wants now is to turn the page, forget and move on.

This is precisely what international organizations are seeking to prevent. When Russia joined the Council of Europe and the Organization for Security and Cooperation in Europe, it undertook clear and binding commitments on human rights. The OSCE Moscow Document stipulates that “issues relating to human rights, fundamental freedoms, democracy and the rule of law . . . are matters of direct and legitimate concern to all participating states and do not belong exclusively to the internal affairs of the state concerned.” At its annual session last month, the OSCE Parliamentary Assembly called on the Russian authorities “to undertake a new, full and thorough investigation into the February 2015 assassination of Boris Nemtsov . . . including the vigorous prosecution of those who ordered or facilitated the crime, and to co-operate with the OSCE Parliamentary Assembly and the Parliamentary Assembly of the Council of Europe in regard to their ongoing interest in this case.”

The Kremlin’s response, at least with regard to the Council of Europe, came in the letter to Vishnevsky. As special rapporteur with a mandate to review all aspects of the Nemtsov case, Zingeris was planning an evidence-gathering trip to Russia. But as the Foreign Ministry has made clear, the only officials he will be able to meet there will be the border guards at Sheremetyevo Airport, who will turn him back.

“It is scary to allow an independent investigation of Nemtsov’s murder. The true picture may emerge, the true organizers and masterminds may be named,” Vishnevsky wrote after reviewing the Foreign Ministry’s letter. “One can understand Russian officials and diplomats . . . But this cannot continue indefinitely. The truth will become known all the same.”