One of Japan's worst mass killings in decades, Thursday's attack at Kyoto Animation killed 34 people after a man shouting "die" set ablaze a bucket of petrol he had poured at the building's entrance.

Nineteen of the dead were found piled on top of each other on a stairway from the third floor to a door leading to the roof, with some early reports suggesting it could not be opened from the inside.

But police quoted by NHK national television on Monday said investigations had shown that while the door could be opened from the inside, smoke from the blaze appeared to have spread so fast it overcame the victims before they could do so.

Asked about the door, Kyoto Animation lawyer Daisuke Okeda said there were keys in two places, adding that he was aware of media reports the door was open but he did not know if it was.

"It had a normal door knob so it was not a complicated mechanism," he told reporters.

Junzo Yamamoto, head of the National Public Safety Commission, offered flowers at the site before stepping in to the building's blackened hulk to inspect it.

"The question of how we can prevent this kind of incident is extremely difficult to answer," Yamamoto told reporters. "Before getting there we need to clarify the whole picture."

Of the 26 people whose autopsy results have been released, 20 burned to death, three suffered carbon monoxide poisoning and two suffocated, Kyoto police said. The cause of death of one was undetermined.

Fire officials told a city assembly panel that everything at the building had complied with the fire code and they were investigating why so many lives were lost, NHK said.

The company had run sufficient fire drills and taken all steps required by law, including "hanging walls" designed to stop smoke from rising, it added.