In the F9-029 Amos 6 mission failure at the launch pad, a "fast fire" happened near the 2nd stage of Falcon 9 FT. In F9-020 CRS-7 mission, an explosion happened in flight (again in the 2nd stage) a few moments before 1st stage cutoff. Failure of a strut was the cause of the 2nd stage explosion in CRS-7 mission.
According to Elon Musk, the Amos 6 mission case is considered the most difficult and complex failure they have ever had in 14 years. SpaceX has also requested support from the public with audio, photo or video materials if they have. This means that the investigation process could last longer and be more difficult than was thought previously.
What are the reasons that it is more difficult to find what caused the disintegration of the Falcon 9 FT rocket in the Amos 6 mission?