ST. LOUIS (NewsNation) — Incredible allegations of cheating have rocked the normally proper world of chess.

Magnus Carlsen, the world champion and a player widely considered one of the greatest ever, posted a statement on Twitter in which he said he believed 19-year-old American opponent Hans Niemann “has cheated more — and more recently — than he has publicly admitted.”

Carlsen lost to Niemann two weeks ago at the “over-the-board,” or in-person, Sinquefield Cup in St. Louis, after which Carlsen withdrew from the tournament.

The Norwegian then quit a game against Niemann at the online Julius Baer Generation Cup last week after making just one move.

Niemann, 19, has admitted to cheating virtually but never over the board.

”I’m not going to back down and I’m going to play my best chess here, regardless of the pressure that I’m under,” said Niemann.

The claims against Niemann, however, get more bizarre.

He’s also accused of using a device inserted in his body to receive off-site help.

The accusations have gained enough traction that they’ve led to full-body scanning at Friday’s tournament.

In a statement days before Carlsen’s accusations, the president of the International Chess Federation, Arkady Dvorkovich, said the governing body shared Carlsen’s “deep concerns about the damage that cheating brings to chess” and is prepared to investigate incidents “when the adequate initial proof is provided.”

However, Dvorkovich added about Carlsen’s conduct in leaving a game after one move: “We strongly believe that the world champion has a moral responsibility attached to his status, since he is viewed as a global ambassador of the game.

The Associated Press contributed to this report.