The fire didn't start slowly like most fires usually did, It exploded and spread across the vicinity in an instant.
But even with this, the explosion occurred from a particular point in the kitchen before spreading outwards and this particular point was where the force would be the highest.
When Gustav got to that place, in particular, he met about three people will missing body parts who had received the full force of the explosion.
One of the three people wasn't dead yet because they happened to be a mixed-blood but the other two died instantly. Gustav still brought their body out regardless.
Apart from this, the last two he found had been burnt beyond recognition. Even when Gustav fed them the regeneration pill, they still died in a manner of seconds after he brought them out of the building.
"Gustav, it's not your fault... You really did good in there," Boss Danzo said while squatting in front of Gustav and rubbing his hair.