A man and his wife arrested earlier this year on suspicion of fatally poisoning their 4-year-old daughter, the man's sister and his father, were served a fourth murder warrant on Friday.

Kenichi Hosoya, 43, and his wife Shiho, 38, were rearrested on suspicion of killing Hosoya’s 68-year-old mother Yaeko by making her ingest ethylene glycol, a toxic substance found in antifreeze, in August 2017, Kyodo News reported.

The two were first arrested in February for the suspected murder of their daughter Yoshiki in March 2023. They were served arrest warrants in March this year for allegedly murdering Hosoya's 41-year-old sister, Minako Hosoya, in April 2018. The two were indicted in September for Minako’s murder after six months of psychiatric examinations determined they could be held criminally responsible.

They were served a third arrest warrant in October for allegedly killing Hosoya’s father Isamu, 73, by poisoning him at his home between August 2017 and June 2018 with ethylene glycol. He died in hospital of sepsis on June 5, 2018.

In the fourth charge, the Hosoyas are accused of getting Yaeko to ingest ethylene glycol in August 2017. In September 2017, she had to start undergoing dialysis treatment for a kidney condition. She died on Jan 21, 2018.

Kenichi Hosoya inherited an apartment and land following his father's death. Police suspect he and his father had fallen out over inheritance and management of the family-operated ryokan business.

Examinations of the couple's smartphones and laptops showed records of antifreeze purchases in August 2017, while messages between the couple on the Line messaging app hinted at their involvement in the alleged murder, including texts that Isamu Hosoya "should die."

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 ultimately lead to death.

Police said the Hosoyas have remained silent since their arrest.