TOKYO (AP) — Japanese sumo officials are investigating allegations Tuesday that nine-time grand champion Harumafuji assaulted a lower-ranked wrestler, causing serious head injuries.

Japanese media reported that Harumafuji hit his fellow Mongolian wrestler Takanoiwa in the head with a beer bottle at a party last month, fracturing his skull base and causing other injuries.

Harumafuji, who holds sumo's highest ranking of yokozuna, appeared on Japanese television to acknowledge his role in the incident and express his remorse.

"I sincerely apologize for causing trouble," Harumafuji told reporters in Fukuoka, a southern Japanese city where the Kyushu Grand Sumo tournament is currently being held.

The Japan Sumo Association, which imposes strict rules on wrestlers, said 33-year-old Harumafuji will sit out the 15-day Kyushu event, which started Sunday, while the investigation takes place.

Harumafuji's stable master Seiya Isegahama told reporters that the grand champion would visit the victim's stable later on Tuesday to offer a personal apology.

The news dominated Japanese television talk shows and evening newspapers on Tuesday as the nation expressed its shock at claims against a yokozuna, whose behavior in sports and society is expected to be exemplary.

Born Davaanyam Byambadorj, Harumafuji debuted in 2001 and has won the championship nine times, with his most recent victory at the autumn tournament in September. He was promoted to yokozuna in 2012.

The incident is the latest scandal to rock the sport of sumo in recent years following investigations into hazing and match-fixing.

In 2010, then-yokozuna Asashoryu, also from Mongolia, retired from the sport after allegations he had attacked a man outside a Tokyo nightclub during a tournament. In June 2007, a stable master and his three wrestlers were convicted over a bullying-death of a 17-year-old junior wrestler.

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