Devil,
Nick L. got it pretty good above. Also, crew met all the requirements - and I commend and respect their professionalism during this matter.
As for the NTSB, GP and Geoff are right - the NTSB has a matrix that assists in dictating the level of investigation.
In reference to "substantial structural damage" - it depends how, when, where, etc. the damage was sustained - keep in mind the main trigger for the NTSB is the people side - serious injury & death.
Most of the "substantial damage" was done during recovery. You will get differing definitions on "substantial" but from what I have experienced it is the difference between whether the operator or the manufacturer can fix it - providing the human factor trigger was not hit.
W1