NHK reported that Aoba, the named suspect, had served time in prison for robbery and that he was being treated for an unspecified mental illness. The report, which cited an unidentified source, said he told police he started the fire because Kyoto Animation had "copied his ideas and stolen from his novel".
The studio had received a string of threatening emails before the attack, the report said.
Kyoto Animation's founder and CEO, Hideaki Hatta, told the media on Friday: "There are people who criticize anime works, just as there are those who support them. When there are threats, we consult with the police, but I have no idea what led to this incident."
The blaze struck at a pillar of Japan's anime industry, which is a national obsession and a main cultural export. Kyoto Animation, known by fans as KyoAni, has produced many animation hits and won worldwide acclaim for its skilled drawings.
The hours after the attack saw an outpouring of anger, grief and confusion both in Japan and overseas.
Outside the charred shell of the company's premises on Kyoto's outskirts, crowds of well-wishers left flowers and messages of support for the victims. Many bowed or knelt and prayed as they paid their respects.
A female animation fan told Kyodo News: "I was really quiet in high school, but thanks to Kyoto Animation, I was able to make friends. I owe them so much."
Chinese student Ou Shouming, 23, who is studying Japanese in Tokyo, traveled to Kyoto by bullet train to pay his respects.
"Kyoto Animation's productions are very popular in China-I have known them since I was in junior high school. They make anime and stories that touch your heart," he said, adding that news of the attack was the top trending item on Sina Weibo.
"I can't understand why someone would do this. I want to hear the reason from the criminal," Ou said.
After news of the attack broke, a campaign was launched on the online fundraising platform GoFundMe, raising nearly $100,000 in two hours. To date, the total stands at $1.90 million.
