Tokyo (CNN) Police in Japan are investigating why a man torched a renowned animation studio in Kyoto, leaving 33 people dead in the country's worst mass killing in almost 20 years.

Officers haven't been able to question the 41-year-old suspect, who is in hospital receiving treatment for severe burns sustained when he allegedly poured what appeared to be gasoline on Kyoto Animation and set it on fire.

A witness told police the suspect shouted "Die!" as he spilled the liquid on the building's ground floor Thursday morning.

The man was later found to be carrying a backpack containing several knives, a Kyoto Prefecture police spokesperson said Thursday.

Twelve men, 20 women and another individual whose gender was unknown died in the blaze and 35 were injured.

It was the worst mass killing in the country since a 2001 arson attack on a building in Tokyo's Kabukicho district, which killed 44 people. The death toll from Thursday's fire was greater than the Tokyo sarin gas attack on a subway in 1995, which killed 13.

Explosion-like sound

The fire broke out at about 10:30 a.m. local time on Thursday (9:30 p.m. ET) in the company's 1st Studio building in Kyoto's Fushimi-ku district. Police said a resident reported hearing a sound like an explosion coming from the studio.

Footage from the scene showed thick smoke billowing out of the four-story building, which is located in a residential area several kilometers south of Kyoto Station, as firefighters worked to douse the flames.

Japanese Prime Minister Shinzo Abe tweeted that he was left speechless at the "gruesomeness of the crime," and offered condolences to victims.

The fire started at about 10:30 a.m. local time on Thursday.

Kyoto Animation's CEO Hideaki Yata told reporters the company has received death threats in the past through the mail, which are referred to police. He expressed sorrow that workers in the industry had been targeted.

"They are the people who carry the Japan's animation industry on their shoulders, I can't bear to accept the fact they are being hurt or losing their lives," he said.

Founded in 1981, Kyoto Animation -- known as KyoAni -- produces animations and publishes anime novels, comics and books, according to its website. Among Japan's most well known studios, its most famous works include "Free!", manga series "K-On!", the anime TV adaptation of "Haruhi Suzumiya" and "Violet Evergarden" which Netflix picked up in 2018.

The company's philosophy as posted on its website includes keeping a "humanitarian" corporate culture and believes that "promoting the growth of people is equal to creating the brightness of works."

There was an outpouring of grief on Twitter on Thursday with fans of the studio offering their condolences.