Why investigate 9/11? The simple answer is because the current administration has failed to do so. The official investigation has been handled poorly by any standards and the information released to the American public well after the fact is woefully inadequate.
One of the key problems with the conventional conspiracy theory surrounding 9/11 is the passenger list problem. Most people figure this has already been worked out, but it hasn't, and it points to the complicity between the news media (that usually works this out) and the government (that made it up). In a nutshell, the hijackers are not on any passenger list, and the passenger lists don't square with what was reported. Also, if the hijackers didn't even have tickets, how did they get on the planes? Can you imagine a flight attendant not noticing five middle-easterners sitting on the plane without tickets or assigned seats? Moreover, the coroners report doesn't list any hijackers. Read this treatment of this problem: http://www.rense.com/general15/perplexingpuzzle.htm Until someone can come up with a reasonable solution to the puzzle, I would submit that we should assume that the entire story of the 9/11 hijacking is contrived.