The elite world of international chess competition is in turmoil amid suspicions that the top player’s long-running undefeated streak ended when his opponent cheated.

Magnus Carlsen, who had won 53 straight matches and is the reigning world champion, lost last week to Hans Moke Niemann, a 19-year-old American grandmaster, in the prestigious Sinquefield Cup in St. Louis, then promptly withdrew from the ongoing tournament. The startling move by Carlsen, a Norwegian, spurred speculation among other top players.

“I think Magnus believes that Hans probably is cheating,” said Hikaru Nakamura, an American grandmaster ranked No. 6 in the world, told The Wall Street Journal.

Emil Sutovsky, the general director of FIDE, the international chess governing body, hinted that something was afoot in the tourney, which has a $350,000 purse.

“No matter how his tournaments went, Magnus Carlsen never quit,” Sutovsky tweeted, “He must have had a compelling reason, or at least he believes he has it.”