When Hans Niemann beat Magnus Carlsen, the world chess champion, in the Sinquefield Cup on Sept. 4, he ended Carlsen’s 53-game unbeaten streak in classical over the board tournaments and set into motion a debacle that has turned into one of the biggest chess scandals in years.

The next day, Carlsen withdrew from the tournament, which is an exceedingly rare move, especially among top players in elite events. He also tweeted a cryptic video of Jose Mourinho, the Portuguese soccer manager, saying, "I prefer really not to speak. If I speak, I am in big trouble.” In the video, Mourinho is speaking at a news conference after a game in which his team might have lost because of questionable officiating, so online observers interpreted Carlsen’s post as insinuating that Niemann cheated in some way during the game. A representative for Carlsen did not respond to multiple requests for comment.

Tony Rich, the executive director of the Saint Louis Chess Club, which hosts the Sinquefield Cup, said in a statement, "A player’s decision to withdraw from a tournament is a personal decision, and we respect Magnus’ choice.” The same day, David Sedgwick, an anti-cheating arbiter, requested that the Saint Louis Chess Club add a 15-minute delay on the live broadcast.