TOKYO (AFP) - A 61-year-old man suspected of starting a fire at a Japanese mental health clinic that claimed 25 lives died in hospital on Thursday (Dec 30), local media said.

Mr Morio Tanimoto had been in critical condition due to burns and carbon monoxide poisoning following the deadly attack in Osaka on Dec 17.

“The suspect died today,” an Osaka police investigator who declined to be named told AFP, giving no further details.

Mr Tanimoto, a former patient of the clinic, died without being interrogated by police, said the investigator.

The fire raged for half an hour, gutting the fourth floor of the narrow commercial building where the clinic, which also provided general medical care, was located.

Japanese media said most of the victims were believed to have suffered from carbon monoxide poisoning, and may have been trapped inside the unit with the exit blocked.

Last week, another investigator told AFP that images from a security camera appeared to show Tanimoto holding paper bags as he arrived at the clinic.

The man placed the bags on the floor at the entrance, “then some kind of liquid leaked from the bags”, the investigator said.

“He squatted on the floor and his hands appeared to move, then flames were seen jumping up from the liquid.”

Deadly fires are unusual in Japan, which has strict building standards, and violent crime is rare.

One year ago, a man was charged with murder over a 2019 arson attack on a Kyoto animation studio that killed 36 people, the country's deadliest violent crime in decades.

The attack sent shockwaves through the anime industry and its fans in Japan and around the world.