A man screaming "You die!" burst into an animation production studio in Kyoto and set it on fire early Thursday, leaving at least 30 people presumed dead and a dozen possibly trapped inside.
The blaze injured dozens of others, some of them critically, Japanese authorities said.
The fire broke out in the three-story Kyoto Animation building in Japan's ancient capital of Kyoto, after the suspect sprayed an unidentified liquid to accelerate the blaze, Kyoto prefectural police and fire department officials said.
Dozens of other people could be still trapped on the third floor of the building, he said.
The suspect was also injured and taken to a hospital, officials said. Police are investigating the man on suspicion of arson.
Survivors who saw the attacker said he was not their colleague and that he was screaming "(You) die!" when he dumped the liquid and started the fire, according to Japanese media reports.
Footage on Japan's NHK national television showed gray smoke billowing from the charred building. Other footage showed windows blown off.
"There was an explosion, then I heard people shouting, some asking for help," a female witness told TBS TV. "Black smoke was rising from windows on upper floors, then there was a man struggling to crawl out of the window."
Witnesses in the neighborhood said they heard bangs coming from the building, others said they saw people coming out blackened, bleeding, walking barefoot, Kyodo News reported.
Rescue officials set up an orange tent outside the studio building to provide first aid and sort out the 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, and is known for mega-hit stories featuring high school girls, including "Lucky Star," ''K-On!" and "Haruhi Suzumiya."