A man suspected of torching an animation studio and killing 33 people in Japan's worst mass killing in 18 years had been convicted of robbery and carried out the attack because he believed his novel had been plagiarised, media reports say.
Public broadcaster NHK, which identified the 41-year-old man as Shinji Aoba, citing police, said on Friday he served time in prison for robbing a convenience store east of Tokyo in 2012 and, after his release, lived in facilities for former convicts. He had also received care for mental illness, NHK said.
Thursday's attack in Kyoto targeting well-known animation studio Kyoto Animation killed 33 people while 10 were in critical condition, authorities said.
Most of the dead were killed by carbon dioxide inhalation, NHK said.
It was the worst mass killing in a country with one of the world's lowest crime rates since a suspected arson attack in Tokyo killed 44 people in 2001.
Aoba wheeled a trolley carrying at least one bucket of petrol to the entrance of the building before dousing the area, shouting "die" and setting it ablaze on Thursday, broadcaster Nippon TV said, citing police.
"I did it," Aoba told police when he was detained, Kyodo news said, adding that he had started the fire because he believed the studio had stolen his novel.
Police declined to comment. Aoba was under anaesthesia because of burns he suffered and police were unable to question him, Nippon TV said.
He "seemed to be discontented, he seemed to get angry, shouting something about how he had been plagiarised", a woman who saw him being detained told reporters.
The studio had about 160 employees with an average age of 33, according to its website. That makes it a relatively young company in rapidly greying Japan.
Tributes to the victims lit up social media, with world leaders and Apple's chief executive offering condolences.
Aoba lives in the Tokyo suburb of Saitama, some 480km east of the ancient capital of Kyoto and travelled to the area by train, Nippon TV said.
NHK showed footage of him lying on his back as he spoke to a police officer at the time of his detention, shoeless and with apparent burns on his right leg below the knee.
He had no connection with Kyoto Animation, NHK said.
None of the victims' identities had been disclosed as of Friday. There were 74 people inside the building when the fire started, Kyodo said.
The fire that tore through the building spread so fast not only because it was fuelled by petrol, but because it was funnelled up a spiral staircase and there were no sprinklers to douse it, experts said.
Nineteen of the 33 who died were found on a staircase leading up to the roof from the third floor, bodies piled on top of each other, Kyodo said, citing authorities.
Firefighters arriving soon after the fire began found the door to the roof was shut but could be opened from the outside, Kyodo said.
The victims may have rushed up the stairs to escape the blaze and found themselves unable to open the door, it added.
The fire wasn't put out until early on Friday.
Police investigators searched the smouldering shell of the building for evidence in an investigation that Kyodo said covered suspected arson, murder and attempted murder.
Kyoto Animation produces popular anime series such as the "Sound! Euphonium". Its "Free! Road to the World - The Dream" movie is due for release this month.