Scary that people would actually cause such damage!
Since it was under construction I'd assume the fire happened before the
layers of drywall between units were installed.
@dave31--That is why code requires fire blocking between units. Sorry I disagree
on your assessment.
This is why I would never wish to live in an apartment or a condominium that was
"stick built," that is built of wood studs and joists. Even after the
unit was built, a fire in one of the units would cause the entire structure to
ignite like a match box. The required fire sprinklers might help, but would not
prevent a terrible tragedy.