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(CNN) --
Teenage chess grandmaster Hans Niemann "probably cheated" in more than 100 online games, including some with monetary prizes, according to research by one of the discipline's most popular websites.
Chess.com's 72-page report was published Tuesday, a month after controversy erupted at a top-flight tournament when the world chess champion accused the 19-year-old American of cheating.
According to the report, first referred to by The Wall Street Journal, Niemann privately confessed to cheating the website's chess boss in 2020, for which he was temporarily banned from the platform.
The report says Chess.com closed Niemann's account in September, given his proven history of cheating, suspicions about his recent games, and concerns about his steep and inconsistent rank increase.
"Although we do not doubt that Hans is a talented player, we note that his results are statistically extraordinary," the report said.
Niemann had already publicly admitted to cheating in online games at the ages of 12 and 16, but the investigation alleged that he had cheated more recently.
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Niemann has not responded to CNN's request for comment.
Chess.com has millions of users and hosts more than 10 million chess games a day, according to its owners.
To detect possible cheating, the website uses software that flags suspicious moves by comparing a player's moves with those suggested by a chess engine.
According to the report, less than 0.14% of players cheat on the site.
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Carlsen's accusations
The controversy began last month when world chess champion Magnus Carlsen accused Niemann of cheating at the $350,000 Sinquefield Cup in Saint Louis, Missouri.
"I think Niemann has cheated more, and more recently, than he has publicly admitted," the 31-year-old Norwegian said in a statement posted on Twitter.
"His progress on the board has been unusual, and throughout our game in the Sinquefield Cup I got the impression that he wasn't tense or even fully focused on the game in critical positions, while outplaying me as Black in a way that I think only a handful of players can do it. This game helped change my perspective."
Carlsen withdrew from the tournament after losing to Niemann.
The world governing body for chess, FIDE, announced last week that it is investigating Carlsen's allegations.
Hans Niemann plays in the Sinquefield Cup in Saint Louis, Missouri.
Credit: Saint Louis Chess Club
Board chess is played face-to-face, rather than online.
Carlsen did not elaborate on how Niemann may have cheated.
In an interview with the Saint Louis Chess Club last month, Niemann said he had never cheated in face-to-face games.
"I cheated in random games on Chess.com. I was confronted. I confessed. And this is the biggest mistake of my life," he said.
"And I'm completely embarrassed. I'm telling the world because I don't want any misrepresentations or rumours. I've never cheated at a table game. And other than when I was 12, I've never cheated at a prize tournament."
World chess champion Magnus Carlsen explicitly accuses his rival Hans Niemann of cheating
According to the Chess.com report, cheating in a game on the chessboard could involve "various methods such as: hand signals from a nearby coach or access to a phone in the bathroom, a hidden device in a shoe or a cord or doorbell attached to the body".
The report said that Chess.com had not routinely investigated cheating in over-the-board games, but believed that Niemann's performances in some live games "deserve further data-driven investigation."
"In our opinion, there is no direct evidence to show that Hans cheated in the September 4, 2022 game with Magnus, or to show that he has cheated in other OTB games in the past," the report said.
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