Video appears to show the moment a Russian chess champion smeared poison on pieces that were about to be used by a rival.
Amina Abakarova, 43, has been arrested and suspended after allegedly targeting Umayganat Osmanova, leaving her unwell.
CCTV footage captured ahead of a chess tournament in Makhachkala, southern Russia, last Friday appears to show Abakarova entering the room before the contest began.
She is seen placing a bag under a table, before glancing around her and moving to another table.
There, she appears to take something from her bag before touching the chess set that was to be used by Ms Osmanova.
She then glances around her again before leaving the room.
Abakarova is alleged to have been tampering with Ms Osmanova’s chess pieces, using a vial of what is said to have contained mercury - a metal poisonous to humans.
Ms Osmanova reportedly suffered “severe dizziness and nausea” soon after the game began, and needed medical attention.
She is understood to have later continued participating, and to have won a prize.
“We have video proof showing that one of the players at the Dagestani chess championship, Amina Abakarova from the city of Makhachkala, applied an unidentified substance, which later turned out to contain mercury, to the table where Umayganat Osmanova from the city of Kaspiisk was set to play against her,” Sazhid Sazhidov, Dagestan’s sports minister, said in a statement to the Telegraph.
It is alleged that Ms Abakarova told other competitors she had been taking “revenge” for “nasty things [Ms Osmanova said] about her and her relatives behind her back”, according to the Telegraph.
The incident was reported to the police by a tournament judge.
Mr Sazhidov added: “Like many others, I am perplexed by what happened, and the motives of such an experienced competitor as Amina Abakarova are incomprehensible.
“The actions she took could have led to a most tragic outcome, threatening the lives of everyone who was present, including herself. Now she must answer for what she did by the law.”
Alexander Tkachyov, executive director of the Russian Chess Federation, told Russian state-owned news agency Tass that Ms Abakarova could be banned for life by the organisation if found guilty.
“We are waiting for the investigation from law enforcement bodies,” he said.
“If [Ms Abakarova] is found guilty, our reaction will be stiff, possibly a lifetime ban.”
Malcolm Pain, director of international chess at the English Chess Federation, said he had “never seen anything like this before”.
“This is the first recorded case of somebody using a toxic substance, to my knowledge, in the history of the game of chess,” he told The Telegraph. “This is off the scale.”
