Baburova and the 2004 murder of anthropologist Nikolai Girenko, both the work of right-wing extremists. The perpetrators of certain other killings have been charged and convicted: since 1999 none of the instigators, the men who ordered the killing, have been identified or brought to justice. Political prisoners The right to a fair trial and freedom from political or religious persecution has been violated ever more frequently over the past decade. The numbers reliably considered to be political prisoners have risen sharply in the last four years. In May 2016, the Memorial Human Rights Centre put the total at 89. By May 2017, Memorial considered there were at least 117 political prisoners or prisoners of conscience (66 accused of belonging to the Muslim organisation Hizb ut-Tahrir al-Islami which has been banned in Russia since 2010). Among these prisoners is also human rights defender Emir-Usein Kuku from Crimea who was accused of belonging to Hizb ut-Tahrir although he denies any involvement in this organization. Amnesty International has called for his immediate liberation. At various times those imprisoned have included human rights defenders, journalists like Mikhail Trepashkin, and scientists such as Valentin Danilov. Since 2007, loosely-worded laws against "extremism" or "terrorism" have been used to incarcerate the often youthful activists who have protested in support of freedom of assembly, against the alleged mass falsification of elections in 2011 and, since 2014, against the occupation of Crimea, the conflict in eastern Ukraine and corruption in the highest echelons of the government and State. Political prisoners are often subjected to torture in prisons and penal colonies. On 10 May 2014, Ukrainian filmmaker Oleg Sentsov was arrested in Simferopol, Crimea. He was taken to Russia, where he was sentenced to 20 years in prison for alleged terrorist activities. Amnesty International considered the trial unfair and called for the release of Sentsov. Human Rights Watch described the trial as a political show trial calling for the liberation of the filmmaker. On 7 September 2019 Sentsov was released in a prisoner swap. In May 2018 Server Mustafayev, the founder and coordinator of the human rights movement Crimean Solidarity was imprisoned by Russian authorities and charged with "membership of a terrorist organisation". Amnesty International and Front Line Defenders demand his immediate release. There were cases of attacks on demonstrators organized by local authorities. With the passing of time some of these prisoners have been released or, like Igor Sutyagin,