<<<The TSB issued an advisory Friday on the proper weighing of cargos -- suggesting investigators believe the plane may have been overloaded.>>>
Weight-Theft of Air Cargo
Weight-theft of air cargo is probably difficult to prove because the manifests and loadsheets are always going to appear pukka.
Unfortunately, if the air-crew are the last to know (as in ignorant of it), then they end up taking off overweight. Figures of 20% to
30% of cargo under-statement are mentioned in the article above. They'd probably be significant for a marginal operation that's designedly runway length limited (like out of Halifax carrying a heavy load and some tankered fuel).
Allegedly there were a number of witnesses who heard the aircraft go to max power about midway through their final take-off. That may have been the point of dawning realization - where they became "the last to know".