A 61-year-old man suspected of starting a fire that killed 25 people at a Japanese mental health clinic died in hospital on Thursday, police said.
Morio Tanimoto had been in critical condition, suffering from burns and carbon monoxide poisoning following the deadly attack in Osaka on December 17.
"The suspect died today," an Osaka police investigator who declined to be named told AFP, giving no further details.
Tanimoto, a former patient of the clinic, died before he could be 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 been killed by 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."
- String of assaults -
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 crime in decades.
The attack sent shockwaves through the anime industry and its fans in Japan and around the world.
And in recent months there has been a string of assaults on trains and at train stations, several involving fires and knives.
On Halloween, a man was arrested in Tokyo for attempted murder after he allegedly stabbed a passenger and started a fire inside a moving train, wearing an outfit reminiscent of comic book villain the Joker.
Nobuo Komiya, a professor and criminology expert at Taisho University in Tokyo, told AFP that the reoccurrence of large-scale arson attacks could be partly due to Japan's strict gun laws.
"If a criminal gets angry at something, one of the easiest ways to take violent action in Japan is to start a fire. In the US, the easiest would be using a gun," he said.
"Using knives and swords sets a higher psychological and physical hurdle, as you need to get very close to the target of your attack."