NFPA 921 – 9.3.6 Spoliation of Evidence.
“Once evidence has been removed from the scene, it should be maintained and not be destroyed or altered until others who have a reasonable interest in the matter have been notified. Any destructive testing or destructive examination of the evidence that may be necessary should occur only after all reasonably known parties have been notified in advance and given the opportunity to participate in or observe the testing.”
Glenn Corbett, Professor of Fire Sciences, discusses the destruction of evidence.
NIST states that they used no physical evidence in their investigation
From NIST’s FAQ page:
FAQ 28. NIST’s entire investigation included no physical evidence. How can the investigators be so sure they know what happened?
In general, much less evidence existed for WTC 7 than for the two WTC towers. The steel for WTC 1 and WTC 2 contained distinguishing characteristics that enabled it to be identified once removed from the site during recovery efforts. However, the same was not true for the WTC 7 steel. Certainly, there is a lot less visual and audio evidence of the WTC 7 collapse compared to the collapses of the WTC 1 and WTC 2 towers, which were much more widely photographed. Read More at NIST FAQ page