It was that long? Usually once a smoke detector is activated the building will burn within a couple of minutes, it is too late, a fire in the wall spreads upward into an attic. This is called balloon frame construction.It was so small it didn't set off the smoke detector. But within 15 min after going off the whole top of the building was sending flames 100 feet in the air. Sorry I am not a not a rocket scientist?
So it was burning undetected, smoke concealed and not reaching a detector for some time. Maybe an hour or more. Odor of smoke now reaching the residents. The fire finally broke through into the attic and grew tremendously due to the extra available open air in the attic that will feed the fire. Finally the detector activated but it is too late. Most likely a concealed fire burning for an hour or more.
Hey you are learning. Good job.
The odor of smoke is noticeable long before a fire is detected, located or activates the alarm system.