The repeatedly denied having any inkling of what was going to happen.
What they had, and what they stated publicly that they had had, was a lot of very unspecific pieces of chatter
They had Americans working at flight schools calling them, and warning them that something wasn't right with those muslims who wanted to learn how to fly, but not takeoff or land. One flight school instructor (in Minnesota, I believe) actually suggested to the feds that they could use the planes as missiles.
Another flight school instructor contacted the FAA to complain that one of the muslims didn't speak English well enough to fly (English is the universal flight language), and something might be amiss. So the FAA sent a bureacrat to sit with the student. They suggested that the flight school instructor hire an interpreter to help him. The "student" later piloted a 747 into the Pentagon.
And then there is the Agent Williams memo. And countless other examples.
That they didn't find the needle buried in the huge haystack does not equate to lying.
When they denied ANY prior knowledge, that is lying.
It was more important to them to expand the government than tell the truth.