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The trained bear didn't recognise 28-year-old Valentin Bulich when he came into the enclosure and attacked him, according to a source at the Great Moscow State Circus.
The brutal attack saw the large creature - called Yasha - tear the trainer's scalp off and claw at his body.
Fellow circus workers managed to drag the battered body of the young man out of the bear's cage, but he died in hospital as a result of his injuries some time later.
It had been claimed that Bulich - who had always dreamed of being a bear tamer - had gone into the cage to further hone his skills, but it has since turned out he was actually just going in for a routine cleaning.
He locked the door inside, as per the normal practise to stop the bear escaping, but that only served to trap him in once the bear attacked.
The circus source told local media outlet BAZA Media: "Valentine went in alone - this bear knew him well.
"What happened? He forgot to take off his Covid mask.
"He entered the cage with a mask, and the bear did not recognise him.
"He turned away from the bear with his back, and it immediately attacked him."
The source continued: "Everything looked as if it was a normal routine work.
"He entered the cage, closed it, and turned around in the mask."
Since the death, a full investigation has been launched into the incident and the circumstances and safety measures surrounding it.
The source confirmed that while going into a bear's cage alone is 'a violation of safety rules' it is 'the norm in the circus'.
Senior circus official Tatyana Zapashnaya said: "It is very difficult for us. Everything happened very quickly and unexpectedly.
"This is a disaster that we must deal with. We are very concerned about this incident."
Yasha the bear is well-known inside and outside the circus for performing multiple stunts, including some in which he is trusted well enough to get close to humans.
In August, he was invited onto a football pitch in Moscow to give a 'pep talk' to the team.
However, animal welfare groups object strongly to the use of any animals - let alone large bears - in circuses.
Irina Novozhilova, of animal rights group VITA, said: "No circus conditions will be humane for one simple reason.
"Training goes hand in hand with cruelty. Circuses are always cruel beyond limits, and circuses with animals should be banned."
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