The 72-page report by Chess.com was released on Tuesday, a month after controversy erupted at a top tournament when the world chess champion accused the 19-year-old American of cheating.
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According to the report, first referenced by the Wall Street Journal, Niemann privately confessed to cheating to the website's chief chess officer in 2020, which led to him being temporarily banned from the platform.
The report said Chess.com closed Niemann's account in September given his previous acknowledgments of cheating, suspicions about his recent play and concerns about the steep, inconsistent rise in his rank.
"While we don't doubt that Hans is a talented player, we note that his results are statistically extraordinary," the report said.
Niemann has previously admitted publicly to cheating in online matches at the ages of 12 and 16 but the investigation alleged he had cheated more recently.
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 suspected cheating, the website uses software that flags suspicious moves by comparing a player's moves to those suggested by a chess engine. Fewer than 0.14% of players ever cheat on the site, according to the report.
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Carlsen allegations
The controversy began last month, when world chess champion Magnus Carlsen accused Niemann of cheating at the $350,000 Sinquefield Cup in St. Louis, Missouri.
"I believe that Niemann has cheated more -- and more recently -- than he has publicly admitted," the 31-year-old Norwegian said in a statement posted to Twitter.
"His over the board progress has been unusual, and throughout our game in the Sinquefield Cup I had the impression that he wasn't tense or even fully concentrating on the game in critical positions, while outplaying me as black in a way I think only a handful of players can do. This game contributed to changing my perspective."
Carlsen pulled out of the tournament after losing to Niemann. Chess' global governing body, FIDE, announced last week that it is investigating Carlsen's allegations.
Over the board chess is played face to face, rather than online. Carlsen did not provide details about how Niemann may have cheated.
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In an interview with the St. Louis Chess Club last month, Niemann said he had never cheated in over the board games.
"I cheated on random games on Chess.com. I was confronted. I confessed. And this is the single biggest mistake of my life," he said. " And I am completely ashamed. I am telling the world because I don't want misrepresentations and I don't want rumours. I have never cheated in an over the board game. And other than when I was 12 years old I have never cheated in a tournament with prize money."
According to the Chess.com report, cheating in an over the board setting could involve "various methods such as: hand signals from a nearby coach or accessing a phone in the bathroom, a hidden device in a shoe, or a wire or buzzer taped to the body."
The report said Chess.com had not typically investigated cheating in over the board games, but it believed Niemann's performances in some live games "merit further investigation based on the data."
"In our view, there is no direct evidence that proves Hans cheated at the September 4, 2022 game with Magnus, or proves that he has cheated in other OTB games in the past," the report said.
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