An official with the Kyoto fire department told ABC News that at least 33 people were confirmed dead. Dozens of others were reported injured.
The blaze broke out Thursday morning at Kyoto Animation after a man allegedly burst in and spread an unknown liquid that set the three-story building ablaze, the city's prefectural police said, according to The Associated Press.
The suspect is apparently among those being hospitalized with injuries, the AP reported.
In a post on Twitter, Japanese Prime Minister Shinzo Abe called the fire "too appalling for words."
The blaze, if an arson attack, could be the deadliest mass killing in Japan in three years. The last mass murder was in 2016 when 19 people were stabbed to death at an assisted living center near Tokyo.
ABC News' Morgan Winsor contributed to this report.