Wakashio Togashi, 45, who had been a senior member of the Aum Supreme Truth cult, was found dead in the bathtub at another Aum follower’s house in Tokyo on Saturday, a police spokesman said.
Togashi had served seven years in prison for helping build a plant to produce Nazi-invented sarin gas used by the cult in several attacks.
“We found that he drowned to death, but the group suspects that he died from an accident while going through hot water training,” the spokesman said.
Members of the doomsday cult are supposed to soak for long hours in water at temperatures of about 50 degrees Celsius.
The cult, which was renamed Aleph in 2000, said in a statement: “We have imposed a total ban on hot water training from now on.”
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The Aum Supreme Cult was founded in 1984 combining Buddhist and Hindu mysticism with apocalyptic visions.
The sect spread sarin gas on the Tokyo subway in March 1995, killing 12 people and injuring thousands in an apparent bid to ward off a police raid.
Shoko Asahara, the founder of the Aum Supreme Truth sect, was sentenced to death in February for crimes including the subway attack.
He is appealing against the sentence but his lawyers have tried unsuccessfully to suspend the hearing arguing that the guru is no longer psychologically sound.
In October, four breakaway members of the cult were arrested for allegedly battering a woman to death with bamboo sticks in a Tokyo apartment in an exercise meant to rid her of bad karma.