I think this question needs to be looked at carefully.
1) People who do things for a living _always_ know about many options that outsiders cannot even guess at.
It seems reasonable to assume that the people who guard against terrorism for a living have ways of dealing with serious threats that we can't even guess at.
2) But even if we limit ourselves to accepted options, there seem to be steps that could have been taken.
One simple one seems to have been air marshalls or undercover people on all domestic flights during September. Remember, one man with a gun could have stopped each of those 911 hijackings. That one simple thing -- deploying a few hundred people onto planes for a limited time period -- would have prevented the 911 attacks.
Other measures raise questions. If someone had deployed ground-to-air missles around New York or DC, would _anyone_ have had the political will to use them? (Prior to the 911 attacks, only specialists and people who read Tom Clancy knew about using planes as missles.)
Another question is what about public education. Since Clancy wrote about the scenario, obviously it was something that specialists knew about. Why wasn't there more public education about the possibility of turning planes into missles? If there had been, then missles used in defense of NY might have been acceptable...
Mark W.
So we flood the airlines with air marshalls. The perps watch the news and hear about it, fly, with their legal boxcutters, to California, fly back and wait. They are not on a set schedule for the date of the operation, only within the operation. After all, supposedly the plan was first discovered in some Phillipine thing in '95, if I remember correctly.
Unless we knew who the perps were, just like now, everything else is a matter of when. The level of evidence that would have been required pre-9/11 to convince an agency that what was going to happen was real would almost have to be sufficient to nail the actual perpetrators.