I hate that these kind of stories only come around once a year. But the NY Times is running this article which says the Bush administration was repeatedly warned of an attack by Al-Qaeda instead of a single warning weeks before:

The direct warnings to Mr. Bush about the possibility of a Qaeda attack began in the spring of 2001. By May 1, the Central Intelligence Agency told the White House of a report that "a group presently in the United States" was planning a terrorist operation. Weeks later, on June 22, the daily brief reported that Qaeda strikes could be "imminent," although intelligence suggested the time frame was flexible.

…

Yet, the White House failed to take significant action. Officials at the Counterterrorism Center of the C.I.A. grew apoplectic. On July 9, at a meeting of the counterterrorism group, one official suggested that the staff put in for a transfer so that somebody else would be responsible when the attack took place, two people who were there told me in interviews. The suggestion was batted down, they said, because there would be no time to train anyone else.

I try not to think "what if" or "if only". We have a job to do: rebuild downtown New York; make that skyline a symbol again. Learn from the past, sure, but don't sulk and think of regrets or missed opportunities. Learn from what happened and make things better.