The problem is, it was known that the bridge was structurally unsound thanks to its age, but the elements that corroded and actually caused the damage could not be inspected at all. The report [1] is quite fascinating, the meat is on page 53/54:

> Auf Grundlage der gewonnenen Erkenntnisse und der positiven Berechnungsergebnisse wurde in der Gesamtbetrachtung weder ein akuter Handlungsbedarf festgestellt noch eine Verstärkung als erforderlich erachtet

> (Based on observation results and positive simulations no need to act was derived, nor was an increase in observation deemed to be necessary)

The root cause is deemed to be errors made all the way back during construction, most probably too long exposure of the steel cables to the environment (see page 108).

Only thanks to this desaster the actual failure mode and how to spot it got known in the first place. The report suggests (page 110) that bridges of a similar construction type (and thus, the same weakness) be retrofitted with acoustic monitoring to detect snapping cables.

[1] https://www.dresden.de/media/pdf/Strassenbau/Gutachten-Carol...