Morio Tanimoto had been under investigation on suspicion of arson and murder in the December 17 fire at a mental health clinic in Osaka in western Japan.

He died on Thursday, police said on Friday.

Tanimoto, a clinic patient, was seen on security camera footage. He had bought a large amount of petrol, and the incident drew comparisons to a 2019 arson attack on an animation studio in Kyoto that killed 36 people.

Japan's nationally circulated Yomiuri newspaper said Tanimoto had been in a coma, adding that his death will likely mean much of the crime will remain a mystery.

A retired metal worker, Tanimoto had troubled relations with his family, according to Japanese media reports.

The Osaka fire gutted an eight-storey building, and people were trapped inside, although firefighters put out the blaze within an hour.

It raised serious questions about the safety of the building's design, as there was only one way out, and the elevator and emergency stairs were located outside the clinic.

Among those killed were patients and the head doctor.

The suspect in the 2019 arson at the animation studio, Shinji Aoba, recovered, has been charged and faces trial.

That fire prompted an outpouring of sympathy both from within Japan and abroad, as well as donations for victims and to rebuild the studio. It had destroyed much of the animation material at the famed studio.

Both fires shocked Japan, a relatively low crime nation. The deep conformist tendencies of Japanese society sometimes set off discriminatory attitudes about mental health.

Australian Associated Press