By Steve Jackson
Japan's PM Yasuo Fukuda has called on sumo wrestling authorities to take action, after the high-profile arrest of a coach and three wrestlers.
Sumo wrestlers are widely seen as role models in Japan
The four are alleged to have beaten trainee Takashi Saito to death during a violent practice session.
The 17-year-old collapsed and later died in hospital.
Mr Fukuda has told the sport's governing body to make sure that cases like this could not happen again.
Sumo wrestling is Japan's national sport and those involved are widely seen as role models, and much respected for their honour and humility.
The arrest on Thursday of the Sumo stable master, Junichi Yamamoto, has caused widespread shock.
Many newspapers have run front page pictures of him being driven away by police covering his face with his hands.
WHAT IS SUMO?
Sumo is Japan's national sport, dating back hundreds of years
Two wrestlers face off in an elevated circular ring
They try to push each other to the ground or out of the ring
Wrestlers are ranked, and the highest level is yokozuna
The police allege that Mr Yamamoto ordered several other wrestlers to beat the 17-year old, Takashi Saito, during a training session last June - an allegation he denies.
Investigators initially said his death was caused by an illness, but a recent autopsy found that he had died of traumatic shock.
Young Sumo wrestlers train in tight-knit stables, where they eat sleep and practice together and are sometimes subjected to harsh treatment in the belief that it will toughen them up.
The allegations surrounding Takashi Saito's death have prompted expressions of concern at the highest levels.
The chief cabinet secretary, Nobutaka Machimura, described the case as truly deplorable.
The education minister, Kisaburo Tokai, has warned that Sumo needs to become more accountable to restore its honour.
The Japan Sumo Association apologised and called on those arrested to cooperate with the police.