A man suspected of torching an animation studio in western Japan shouted that he had been plagiarised and appeared to have planned the attack, media said today after a blaze that killed 33 people in Japan's worst mass killing in two decades.

The 41-year-old man "seemed to be discontented, he seemed to get angry, shouting something about how he had been plagiarised", a woman who saw the suspect being detained told reporters.

Also read: Japan Animation Studio: 33 killed in 'arson attack'

The unidentified man shouted "Die!" before dousing the entrance to Kyoto Animation headquarters with what appeared to be petrol and setting it ablaze around 10:30 am (0130 GMT) on Thursday, media reported.

The explosive blaze killed 33 people and another 10 people were in critical condition, authorities said late on Thursday, in Japan's worst mass killing since a suspected arson attack in Tokyo killed 44 people in 2001.

A day later, none of the victims' identities had been disclosed.

"I imagine many of the people who died were in their twenties," said 71-year-old Kozo Tsujii, fighting back tears after laying flowers near the studio in the rain on Friday morning. He said he drives by the studio on his daily commute.

"I'm just very, very sad that these people who are so much younger than me passed away so prematurely," he said.

Police searched through the smouldering shell of the building for clues on Friday.

A man resembling the suspect went to a petrol station on Thursday with two 20-litre cans, Japanese media said. Two cans, a rucksack and a trolley were found near the site, and television footage showed what appeared to be five long knives laid out by police as possible evidence on the ground outside the three-storey building.

The suspect had no connection with Kyoto Animation and his driver's licence listed an address in Saitama, a northern suburb of Tokyo, public broadcaster NHK said. Little else was known about the man, who is under police supervision with serious burns to the face and legs, media reports said.

NHK showed footage of what it said appeared to be the suspect lying on his back on the ground as he spoke to a police officer, shoeless and with what appeared to be burns on his right leg below the knee.

Kyoto Animation, located in a quiet suburb about 20 minutes by train from the centre of Japan's ancient capital, produces popular "anime" series such as the "Sound! Euphonium".

Its "Free! Road to the World - The Dream" movie is due for release this month.

The studio sits between two train lines close to a stream amid houses and small apartment blocks, a few stores and a taxi company parking lot. The area was cordoned off on Friday morning. A few of the original 30 or so fire trucks remained in the streets but with their lights off. Some firefighters appeared to still be dousing the smouldering building.

Jun Shin, a 30-year-old Chinese man living in nearby Osaka, came to the site on Thursday night to lay flowers near the burnt-out office and say a prayer.

"I am an anime fan," Jun, an information-technology worker, told Reuters. "I have watched animation since I was a student, and this was a terrible event, I just want to come and mourn. It left me speechless."