Investigative sources say the man suspected of starting a deadly fire at a clinic in the western Japanese city of Osaka has died. His death will make it harder to determine the motive for the arson attack.

The sources say Tanimoto Morio died on Thursday evening at the hospital where he had been in critical condition after suffering carbon monoxide poisoning in the incident.

The 61-year-old man is believed to have bought gasoline before spraying and lighting it at the mental health clinic on the fourth floor of a building on December 17. Twenty-five people were killed and another remains in serious condition.

Police have been investigating Tanimoto, who was a patient of the clinic, on suspicion of arson and murder.

He is believed to have premeditated the arson attack and carried it out with intent to kill. But police say they have not found any evidence of trouble between the clinic and the suspect that could have led to the incident.

Police found a memo bearing the handwritten words "arson-murder" during a search of a property related to Tanimoto. They also found newspaper clippings on past arson cases in Japan, including the 2019 attack at a studio of the Kyoto Animation production company that left 36 employees dead and 32 others injured.