Ugh some people are just evil. :|
I hope they will get to save that arsonist and bring him to court.
Based on SilentPanda's post, I guess the whole entrance area was burned, and the people upstairs were trapped; I'm curious as to why the victims on the 2nd floor couldn't go to the balcony and escape from there, maybe the fire/smoke got there too quick. The victims on the 3rd floor probably tried to get to the roof and it was either locked or the fire/smoke got them too :\
Here is the transcript of the broadcast by the expert on NHK
NHK had former fire investigator for police and CSI Professor Haeito Keizo explain
"1階の窓が壊れて飛散していることから、1階部分で爆燃現象、爆発現象が起きたと思われる。2階と3階は外壁が焼け落ちたような状況がみられるため、非常に温度の高い状況が、それも短時間で発生したと推定される。これらの状況からガソリンのような液体がまかれて着火し、急速な火災が起こったと思われる "
From the first floor window and debris spread, it is concluded that the first floor had an explosion occur, and the extensive damage to the outer wall of the 2nd and 3rd floor, show that it was very high temperture, so it is likely that an liquid like gasoline or other was used to start and make the fire bigger"
通常の火災は室内の温度が1000度以上は上がらないが、ガソリンは場合によって1500度まで上昇する。これは鉄が溶けるような熱さで火炎を浴びただけで息ができなくなる。爆発現象が起きた1階では、状況が分からないまま人々が火炎に包まれ、2階と3階にいた人たちは爆発音は聞いていると思うが、何がなんだかわからないまま炎と煙に襲われて、逃げ場を失ったと思われる
Normal fire inside a building don't reach temperature over 1000 degree, but gasoline fire can reach over 1500 degree, able to melt metal at that temperture it is impossible to breath, people on the first floor was perished quickly, 2nd and 3rd floor heard the explosion, but before had time to reach, the fire and smoke came, and they didn't had time to run/react.
ガソリンは着火と同時に火炎が数メートル上がるなど、通常とは比較にならないスピードで炎が広がるので、おそらく火災報知器が鳴る時間も、防火扉が作動する時間もないぐらい勢いよく燃え広がったと思われる。火が出てからでは逃げるのは困難で、建物内の入退室の管理や荷物のチェックなどのセキュリティー強化が必要だ」と指摘しています。
Due to the unatural speed and spread of the fire, likely rising a few meter after igniting so spreading very fast, there wasn't time for the fire alarm and fire door to activate, making escape impossible, the only secure way now is to increase checking of personal item and coming in and out of building.