Russia is responsible for the poisoning and attempted killing of jailed opposition figure Alexei Navalny, two independent UN rights experts said on Monday, issuing an "open call" for an international investigation into the incident. Special Rapporteurs Agnès Callamard and Irene Khan believe the politician was poisoned to send a "clear, sinister warning" to anyone wanting to criticize the government. Navalny fell violently ill on a domestic flight from Siberia to Moscow last August. He was later flown to Germany where toxicology reports determined he had been poisoned with Novichok, a Soviet-era nerve agent. On returning to Moscow last month, he was imprisoned for violating a sentence for alleged embezzlement. "It is our conclusion that Russia is responsible for the attempted arbitrary killing of Navalny", said Callamard, UN Special Rapporteur on extrajudicial, summary or arbitrary executions, speaking during a press conference in Geneva. Part of a larger trend The independent experts were appointed by the UN Human Rights Council and are not UN staff nor are they paid by the orgnaization. They conducted a four-month investigation into the Navalny case and wrote to the Russian authorities last December but never received a response. Their letter was made public on Monday, in line with Council provisions. They said only Russia is known to have developed, stored and used Novichok. A novel version was used against Navalny, suggesting further development of the toxin. It was also "very unlikely" that non-State actors would have the capacity to develop or use the nerve agent, or that private buyers would have the expertise to properly handle it. "It is also the findings of our work that the poisoning and attempted killing of Navalny, along with the lack of investigation and the denying narratives, are part of a larger trend, ongoing over several decades, of arbitrary killings and attempted killings, including through poisoning, by the Russian authorities of journalists, critics and dissidents and are therefore consistent with an overall pattern of modus operandi," Callamard added. Sowing fear in opponents Navalny has long been a staunch critic of the Kremlin who repeatedly denounced corruption, said Khan, the UN Special Rapporteur on the promotion and protection of the right to freedom of opinion and expression. He had suffered two previous chemical attacks prior to the Novichok poisoning, as well as physical attacks, threats, harassment, surveillance and criminal sanctions. "The motive of the poisoning, preceded by a long history of attacks, harassment and trumped up charges, was both to violate the human rights of an individual but also to knock out a political opponent," she said. "And we believe that there might be a broader purpose to the poisoning. Novichok was chosen precisely to cause fear. And we believe that the poisoning of Navalny might have been carried out deliberately to send a clear, sinister warning that this would be the fate of anyone else who might criticize and oppose the government." — UN News