Two doctors who were indicted last year over the consensual killing of a terminally ill woman in Kyoto were indicted again on Thursday on a charge of murdering the father of one of them 10 years ago, prosecutors said.

The doctors -- Yoshikazu Okubo and Naoki Yamamoto, both 43 -- as well as Yamamoto's mother Junko, 76, were indicted over the death of Yamamoto's 77-year-old father Yasushi on March 5, 2011 in an apartment in Tokyo, they said. He was cremated without an autopsy.

The two doctors were indicted in August last year for allegedly giving a 51-year-old woman suffering from amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, a lethal dose of a sedative drug at her home in Kyoto in November 2019. Before her death, the woman had transferred money to Yamamoto's bank account.

The Kyoto District Public Prosecutor's Office did not reveal whether the three have admitted or denied the charges or what roles each of them may have played in the death of the father.

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The prosecutors also said they are uncertain about the precise cause of Yamamoto's death.

Police served the doctors with fresh arrest warrants and arrested Junko Yamamoto on May 12. They discovered emails between the three discussing the cremation of Yasushi Yamamoto and how to write his death certificate.

He had been hospitalized in Nagano Prefecture for a mental disorder, but the three had him discharged on March 5, 2011, for transfer to a different hospital, according to investigative sources. He was allegedly killed later that day.

His death certificate, which said he died suddenly due to disorders in the heart and blood vessels, was signed by another doctor, a mutual acquaintance of Naoki Yamamoto and Okubo.

But that doctor told the police he had not signed any death certificate, the sources said.