The majority of those who died in the arson attack on Kyoto Animation on Thursday were in their 20s and 30s, public broadcaster NHK said on Saturday.

More clues as to why the fire spread so quickly and took so many lives, as well as details about the suspect, are also emerging.

Thirty-four people are now confirmed to have died, with another succumbing to their injuries overnight on Friday, and the same number are still in hospital.

Kyoto Animation’s president, Hideaki Hatta, who founded the company with his wife Yoko in 1981, said many who died were young women and some had joined in April, the traditional time for new employees to start work in Japan.

“And on the eighth of July, I gave them a small, but their first, bonus. People who had a promising future lost their lives,” Hatta told Reuters. “I don’t know what to say. Rather than feeling anger, I just don’t have words.”

KyoAni, as its fans call it, is known for its many female animators and storytellers, who create anime with female characters in an industry that is relatively male-dominated.

One reason for the high death toll in Thursday’s fire may have been the spiral staircase in the centre of the building acting like a funnel for the fire that ripped through the studio. There were also no sprinklers in the building to slow down its rapid spread, though they were not required to be installed in the 700 sq metre building.

The building passed a fire safety inspection in October last year.

Many of the bodies were found on the upper part of the staircase, which led to the roof, suggesting many were overcome by heat, flames or smoke as they tried to escape, Hiroyuki Sakai, the deputy police chief of Fushimi ward, told the Guardian on Friday evening.

Nobody made it to the roof, which appeared to be relatively unscathed by the fire. Sakai was unwilling to speculate as to whether the door to the roof had been locked or the victims simply did not have time to get there. The few who did manage to escape did so by clambering out of windows.

Shinji Aoba, whom the police named on Friday as the sole suspect, is alleged to have entered the building through the front door, carrying 40 litres of petrol in a bucket-like container, said Sakai.

The suspect is believed to have wheeled it on a trolley after transferring it from two 20-litre cans he had bought from a petrol station nearby. The petrol ignited so fast that it sounded like an explosion to people in the quiet residential streets around the studio.

The suspect has been transferred to a hospital specialising in burns and is understood to still be unconscious, though he was talking when taken into custody, complaining about plagiarism, according to witnesses.

Aoba previously served a prison sentence for robbing a convenience store using a knife, according to reports in the Japanese media, and had threatened neighbours in the apartment block where he had until recently been living in Urawa, north of Tokyo.