A couple suspected of fatally poisoning their 4-year-old daughter and the husband's sister were served fresh arrest warrants on Friday for allegedly killing the husband's father six years ago, Tokyo police said.

The third arrest warrants for Kenichi Hosoya, 43, and his wife Shiho, 38, state they are suspected of murdering his 73-year-old father, Isamu Hosoya, by poisoning him at his home between August 2017 and June 2018 with ethylene glycol, a toxic substance found in antifreeze. He died in hospital of sepsis on June 5, 2018.

The two have remained silent about the latest allegation, according to the police.

Kenichi Hosoya inherited an apartment and land following his father's death. Investigators suspect the father and the couple were in a dispute over inheritance and management of the family business.

They determined that Isamu Hosoya likely consumed ethylene glycol after analyzing his blood tests and medical records.

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."

Investigators are also looking into Kenichi Hosoya's mother's death in January 2018. His parents and sister all died in the first half of 2018.

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.

The two were first arrested in February for the suspected murder of their daughter Yoshiki in March 2023. They were served fresh arrest warrants in March this year for allegedly murdering Kenichi Hosoya's elder 41-year-old sister, Minako Hosoya, in April 2018.

The two were indicted in September for the suspected murder of Minako Hosoya after six months of psychiatric examinations to determine whether they could be held criminally responsible.

The investigation into their daughter's death is ongoing.