The blaze in Kyoto injured another 36 people, some of them critically, Japanese authorities said.
Survivors who saw the attacker was screaming when he dumped the liquid and started the fire, according to Japanese media reports.
Most of the victims were employees of Kyoto Animation, which does work on feature films and TV productions but is best known for its mega-hit anime stories featuring high school girls. The tales are so popular that some of the places depicted have become pilgrimage sites for fans.
The fire broke out in the three-storey Kyoto Animation building in Japan’s ancient capital after the suspect sprayed an unidentified liquid to accelerate the flames, police and fire officials said.
One person died of severe burns, said fire department official Satoshi Fujiwara. Most of the 10 seriously injured had burns, as well.
Japanese media reported the fire might have been set near the front door, forcing people to find other ways out.
Firefighters found 33 bodies, 20 of them on the third floor and some on the stairs to the roof, where they apparently collapsed, Kyoto fire official Kazuhiro Hayashi said. Two were found dead on the first floor, 11 others on the second floor, he said.
The suspect was also injured and taken to hospital, officials said. Police identified him only a 41-year-old man who was not a company employee. They gave no immediate details on the motive, but are investigating him on suspicion of arson.
Footage on Japan’s national television network, NHK, showed grey smoke billowing from the charred building. Other video clips showed windows blown off.
NHK footage also showed sharp knives police had collected from the scene, though it was not clear if they belonged to the attacker.
“There was an explosion, then I heard people shouting, some asking for help,” a woman told TBS TV in Japan. “Black smoke was rising from windows on upper floors, then there was a man struggling to crawl out of the window.”
Witnesses in the neighbourhood said they heard bangs coming from the building, and others said they saw people coming out blackened, bleeding and walking barefoot, Kyodo News reported.
“My heart is in extreme pain. Why on earth did such violence have to be used?” company president Hideaki Hatta said, adding the company had received death threats in the past.
Japanese Prime Minister Shinzo Abe confirmed there was a large number of casualties and expressed his sympathies in a post on Twitter.
Rescue officials set up an orange tent outside the studio building to provide first aid and assist those who were injured.
Fire department officials said more than 70 people were in the building at the time of the fire and many of them ran outside.
Kyoto Animation, better known as KyoAni, was founded in 1981 as an animation and comic book production studio. It is known for massive hits such as “Lucky Star,” “Eiga K-On!” and “The Disappearance of Haruhi Suzumiya.”
The company does not have a major presence outside Japan, though it was hired to do secondary animation work on a 1998 “Pokemon” feature that appeared in U.S. theatres and a “Winnie the Pooh” video.
With files from The Associated Press