A document paxvector found at the National Archives tells us how the 9/11 Commission’s investigation of the Environmental Protection Agency (EPA) got started. The commission’s final report did not mention the EPA in its main text, but included a long endnote about it (No. 13 to Chapter 10). The document paxvector found comprises minutes from a commission meeting in the autumn of 2003, which has been posted to the 9/11 Document Archive at Scribd. They say:
EPA/CEQ Investigation. Commissioner Ben-Veniste inquired about the status of an offer to receive pro-bono work from a private law firm in order to investigate allegations of White House interference with EPA health warnings after the 9-11 attacks. The General Counsel stated that he was reluctant to use private law firms to conduct investigative work for the Commission, and furthermore, that the matter was a relatively small investigative piece that he believed could be handled by Commission staff. The Executive Director and General Counsel agreed to develop a plan on how the investigation would be staffed.
You can find that on page 17 of the .pdf file. The General Counsel was Daniel Marcus, the Executive Director was Philip Zelikow. This indicates that the commission may not have investigated the air quality and controversial statements made by the EPA had it not been for the law firm’s offer. AFAIK the commission’s investigation of this was led by Barbara Grewe of the Special Projects team.
All the documents we find about the EPA angle of the commission’s work are grouped here (not many yet, but I’m sure there must be more).