The independent 9/11 commission, formally known as the National Commission on Terrorist Attacks Upon the United States, performed best on the issues it investigated firsthand, which largely were the U.S. government's actions and inactions. Most of its 1,200 interviews dealt with this subject. For information on the plot itself, the commission dealt primarily with reports of investigations by others.

That other reporting by necessity relied on sources of varying credibility. The account of the origin and details of the hijack plot itself come almost entirely from hostile interrogations of two men — Mohammed and one of his deputies, Ramzi Binalshibh, both of whom are in U.S. custody, but neither of whom has shown much willingness to talk about matters that might implicate others.

Also, according to footnotes in the commission report, much of the information on the personalities of the lead hijackers comes from a single German source, a Hamburg teenager who knew the hijackers but did not speak Arabic.

The most readily accepted explanation of the Maine trip is that Atta thought he would reduce his exposure to security by going through a smaller airport, and Portland was the nearest airport with regular service to Boston. The opposite appears to have happened. Rather than reducing his security exposure, Atta doubled it, passing through security checkpoints in Maine and in Boston.

Different hijackers made numerous trips to Las Vegas. Again, there is no evidence that they met other parties there, but no compelling explanation of why they went or what they did there.

http://story.news.yahoo.com/news?tmpl=story&cid=2026&ncid=2026&e=2&u=/latimests/20040726/ts_latimes/questionspersistdespite911investigations