A couple arrested on suspicion of fatally poisoning their 4-year-old daughter were served fresh warrants by Tokyo police on Wednesday for allegedly killing the husband's 41-year-old sister six years ago in a similar way, investigative sources said.
Kenichi Hosoya, 43, and his wife Shiho Hosoya, 37, are suspected of murdering his elder sister Minako Hosoya around April 29, 2018, by poisoning her with ethylene glycol, a toxic substance found in antifreeze.
Kenichi Hosoya was described by investigators as saying he "cannot say anything right now" about the new allegation while Shiho Hosoya was said to be keeping silent.
They were arrested on Feb. 14 for allegedly killing their daughter Yoshiki in March 2023 after traces of ethylene glycol and large amounts of olanzapine, a medicine prescribed for mental disorders, were found in her body.
Kenichi Hosoya's parents and the elder sister all died in the first half of 2018. His father's death in June of that year led to him becoming president of the family business, a Japanese-style inn in Tokyo's tourist-friendly Asakusa district.
Investigators suspect Kenichi Hosoya was in a dispute with his sister over the inheritance and management of the family business.
In addition to Yoshiki, the couple has two other children. In March 2019, police referred the three to the care of a child welfare center after Shiho Hosoya was arrested for setting fire to clothing at home, but they were all returned by September after prosecutors decided not to indict her over the incident.
In the case of Minako Hosoya's death, the couple is suspected of causing her to consume ethylene glycol around April 2018, leading to her death the same month, the investigative sources said.
Bodily samples from an autopsy conducted on Minako Hosoya at the time of her death were reexamined after Yoshiki's death and showed traces of calcium oxalate, a compound produced upon ingestion of ethylene glycol.
Ethylene glycol is a colorless, odorless liquid with a slightly sweet taste that easily dissolves in water. When consumed in large quantities, it can cause damage to the central nervous system and kidneys and lead to death.
Analysis of the couple's confiscated smartphones showed records of multiple antifreeze purchases in April 2018.
People who knew Minako Hosoya described her as a reserved person who had expressed concern about her younger brother's spending habits, and that she had opposed his marriage to Shiho Hosoya.
One acquaintance of hers, a woman in her 50s, said that before her death, she was helping her father run the family business and that Kenichi Hosoya had complained to people around him of being repeatedly admonished by his older sibling.
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