Carlsen quit the tournament and tweeted, "I've withdrawn from the tournament. I've always enjoyed playing in the @STLChessClub, and hope to be back in the future."
The controversy got bigger because, according to the Sunday Morning Herald, Carlsen's used a move called the "Fianchetto variation". It allowed him to take Niemann's bishop. However, Niemann used a rare counter move allowing him to win Carlsen's pawns and ultimately win the match.
Justifying his move, Niemann said he watched footage of Carlsen from an earlier match where he used the same move. But it seemed difficult to imagine Carlsen losing to the lowest-ranking grandmaster in the tournament.