It's Japan's worst case of arson in decades and among the highest mass casualty events in its recent history. 

The three-story building was quickly engulfed after a man sprayed a flammable liquid onto it while reportedly shouting, "You die!" according to local media reports. Many among the deceased had been trapped on the top floor and perished attempting to to get to the roof. 

Police arrested a 41-year-old man who had shouted "die" as he poured what appeared to be petrol around the three-story Kyoto Animation building shortly after 10 a.m. — Reuters

The suspect has been detained and is being treated for injuries at a hospital. Early reports didn't give a motive or were able to identify whether he had been an employee of Kyoto Animation Co., possibly disgruntled or engaged in revenge attack.