Police said Wednesday they have launched a probe into sumo grand champion Harumafuji's alleged assault on fellow Mongolian competitor Takanoiwa in late October that left the victim with serious injuries.

The investigation into the drunken brawl began after police received a damage report from the victim, who was hospitalized for five days. On Tuesday, Harumafuji offered an apology but did not elaborate on the incident.

According to sources familiar with the situation, Harumafuji, a nine-time tournament winner, was drinking with a group of around 10 sumo wrestlers in late October during a regional tour in Tottori, western Japan, when the yokozuna, angered by Takanoiwa's attitude, struck him with a beer bottle.

Harumafuji got angry as Takanoiwa looked at his smartphone while being scolded for his behavior, and punched him up to 30 times in addition to the beer bottle strike, the sources said.

Takanoiwa, the eighth-ranked maegashira, was diagnosed with a concussion, a fracture at the base of his skull and a cerebrospinal fluid leak among other injuries, and hospitalized from Nov. 5 to 9. He has been unable to compete in the ongoing Kyushu Grand Sumo Tournament.

Following the revelation of the incident, Harumafuji pulled out of the tournament.

Mongolian yokozuna Hakuho and Kakuryu, sekiwake Terunofuji and some Japanese sumo wrestlers were among the group drinking with the two, the sources said.

The incident sent shock waves across Japan, reminding fans that the ancient sport has yet to escape its reputation tainted by scandals over match-fixing, violence and bullying.