The case of a now-deceased man was referred to prosecutors Wednesday for suspected murder and arson at an Osaka mental health clinic last December that resulted in the deaths of 26 people and the suspect, police said.

The investigation into the suspected arson on Dec. 17 has come to an end, with prosecutors expected to drop the case after the suspect, 61-year-old Morio Tanimoto, died before police were able to question him.

Photo taken March 16, 2022, shows flowers placed in front of a multiple tenant building hit by a deadly fire in December 2021 in Osaka. (Kyodo)

Tanimoto is suspected of spraying gasoline and starting a fire at the clinic, killing 26 people, including regular visitors to the facility and its director, Kotaro Nishizawa, 49. He is also suspected of igniting a fire at a house he was living in just before the incident.

Tanimoto had been a patient at the clinic in the western Japan city's Kita Ward since 2017 and visited there a total of 114 times. He died on Dec. 30 after being taken to a hospital from the clinic in critical condition.

The police found from data on his smartphone that he was looking into "extended suicide," in which individual who wishes to die commits mass murder, in October last year, investigative sources said.

At the time of the incident, he had no money in his bank account and no log of phone calls with acquaintances.

On the day of the incident, a number of people visited the clinic for a scheduled group therapy session to help individuals on leaves of absence to return to work.

Data on the smartphone's schedule app indicated he started checking the flow of patients in June last year, the sources said.

Police believe Tanimoto was planning an arson attack on a Friday when the clinic is typically crowded.

His motive remains unclear but the police suspect he felt alone and wished to die by suicide.

Related coverage:

Death toll of victims from Osaka mental clinic arson in Dec. rises to 26

Suspect in fatal Osaka mental clinic fire dies