Of those who died, 19 were found piled on top of each other on a stairway.

Smoke spread so quickly through a burning Japanese animation company that a majority of victims who had tried to flee through a rooftop door were unable to open it before perishing, Japanese media said on Monday.

The attack at Kyoto Animation on Thursday, in which a man shouting "Die!" poured a bucket of gasoline at the entrance to the building and lit it, took 34 lives and ranks as one of the worst mass killings in Japan for decades.

Of those who died, 19 were found piled on top of each other on a stairway between the third floor and a door 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 had apparently spread so fast that the victims were overcome before being able to do so.

Kyoto police said on Monday that of the 26 people whose autopsy results have been released, 20 burned to death, three died of carbon monoxide poisoning and two suffocated. The cause of death of one remained undetermined.

On Monday, scaffolding had been erected by the first floor of the building whose outside walls were charred black. Aluminium windows were burnt away on the second storey.

Experts said that a spiral staircase near where suspect Shinji Aoba, 41, allegedly lit the fire acted as a chimney to funnel the smoke upwards through all three storeys of the building. Survivors have described a "dark mushroom cloud" of smoke pouring up the staircase.

"I heard voices arguing on the first floor, then within 15 seconds black smoke welled up from the staircase," one survivor was quoted as telling NHK.

Police quoted by NHK said a second set of stairs, on which many of the victims were found, also may have had a similar effect, meaning that the victims were rapidly overcome by smoke.

The company late on Sunday issued a statement saying that the tragedy had left them at a loss.

"All of these people were our talented, precious colleagues. Both for us here as well as the animation industry as a whole, this is a huge blow," the statement added.

Late on Saturday, police issued an arrest warrant for Aoba, a loner from a city near Tokyo who is in hospital with serious burns. Police plan to arrest him once he recovers.