The National Transportation Safety Board (NTSB) is an agency responsible for investigation of accidents involving aviation, highway, marine, pipelines and railroads in the United States.
It's sometimes asked why the NTSB didn't lead the investigations at the 9/11 crash sites, however the code establishing their authority (TITLE 49 > SUBTITLE II > CHAPTER 11 > SUBCHAPTER III > § 1131) explains that the FBI have "investigative priority" if "circumstances reasonably indicate that the accident may have been caused by an intentional criminal act" (Source).
The NTSB did assist the FBI in a number of areas, though, and many of their documents and analyses have been made public (see below).
The NTSB have released the following 9/11-related documents.
- ATC Report
- Flight Path Study
- Radar Data Impact Speed Study
- Video Data Impact Speed Study
- Debris Trajectory Study
The following Payne Stewart documents shed light on the time it took to intercept his Lear Jet.