Shinji Aoba, 41, suspected of torching an animation studio and killing 33 people, was previously convicted for robbery and treated for mental illness, broadcaster NHK reports.

Smoke billows as fire rages at Kyoto Animation's three-story building in Kyoto, western Japan. July 18, 2019.
Smoke billows as fire rages at Kyoto Animation's three-story building in Kyoto, western Japan. July 18, 2019. (AP)

The man suspected of torching an animation studio and killing 33 people in Japan's worst mass killing in two decades had been convicted for robbery and treated for mental illness, public broadcaster NHK said on Friday.

The suspect, whom police named as Shinji Aoba, 41, was sentenced to 3.5 years for robbing a convenience store in 2012, NHK said. Police officials in Chiba Prefecture, east of Tokyo, declined to comment on the verdict.

After his release, Aoba spent some time in facilities for former convicts. He had also received treatment for mental illness from caretakers who visited his home, NHK said.

At least 10 people were left in critical condition.

'Angry' attacker

The attacker had been described by one witness as "angry" as he poured what is believed to be petrol around the three-storey building.

He allegedly shouted "die" and claimed his work had been plagiarised but did not elaborate.

Earlier in the day, the suspect was spotted on CCTV arriving at a petrol station with two 20-litre containers that were said to be among the items found next to the torched building during the police investigation.

A backpack, trolley and what appeared to be five long knives were also discovered.

Kyoto Animation, better known as KyoAni, was founded in 1981 as an animation and comic book production studio, and is known for mega-hit stories featuring high school girls, including "Lucky Star," ''K-On!" and "Haruhi Suzumiya."

Source: Reuters