A doctor who treated Alexei Navalny after he was poisoned with a Novichok nerve agent has gone missing, according to Russian police.
Alexander Murakhovsky, 49, is believed to have disappeared during a hunting trip in a forest in the Omsk region of Siberia on Friday.
Witness reports suggested that he was last seen leaving the village of Pospelovo on an all-terrain vehicle (ATV).
The ATV was found 6.5 km from the hunting base on Sunday, according to the regional ministry of internal affairs.
Local police, officers from the national guard, hunting inspectors and local residents have all joined the search.
Helicopters and drones have also been deployed because of the "difficult-to-pass swampy areas of the terrain," said the ministry.
Mr Murakhovsky was the head doctor at the hospital in Omsk that treated Mr Navalny after he collapsed on a flight in Russia in August last year and later told a press conference: “We saved his life with great effort and work".
He was subsequently promoted to the post of regional health minister.
Two other doctors at the same hospital in Omsk have recently died, according to notices on its website. The deputy chief physician for anesthesiology and resuscitation "suddenly passed away" on 4 February, while the 63-year-old head of the department of traumatology died on 24 March, supposedly after suffering a stroke.
Another doctor, Anatoly Kalinichenko, who told reporters that no traces of poison had been found, left his post as chief medical officer at the hospital in October.
Alexander Murakhovsky speaks to the media after treating Alexei Navalny in August 2020
Following his treatment in Omsk, Mr Navalny, a prominent critic of Russian president Vladimir Putin, was airlifted to Germany for further treatment and laboratory tests in Europe, confirmed by the global chemical weapons watchdog, later established that he had been poisoned with a Novichok nerve agent.
Novichok came to international prominence in March 2018 when it was identified as the substance used to poison former Russian spy Sergei Skripal and his daughter Yulia in Salisbury.
Russian authorities have repeatedly rejected any suggestion that they tried to kill Mr Navalny.
The dissident was arrested in January when he returned to Russia from Germany and was sentenced to two-and-a-half years in prison for violating the terms of his probation in relation to a 2014 embezzlement conviction that Mr Navalny has rejected as fabricated. Last month he staged a three-week hunger strike to demand proper medical care.