Informative commentary by NIST and Cryptome here:
WTC Attack September 11, 2001 from New York Police Helicopter
Video obtained by an FOIA request from a person to the National Institute of Standards and Technology (NIST.gov) who directed NIST to send it to Cryptome.org. NIST investigated the WTC tower collapses and collected large amounts of evidence from officials and individuals. While much of this evidence has been released to the public, more is still withheld.