Your the one with the problem, you won't accept any other answer than the government failed to do something. In fact you seem to refuse to blame the terrorists. It was the law that the CIA and FBI could not share info, Democrats made that law and they defended it, many of them still want that law to be remade, although it was finally undone after 9-11.
It bears remembering that while there may have been "threats" of hijacking suicide airplanes and crashing them into buildings, there were more threats of bombing tunnels, bombing bridges, blowing up ships, crashing the internet, destroying bank records, bombing planes in flight, taking planes down with stingers, hitting bus stations, blowing up rail lines, taking down power transmission towers, sinking ferry boats, poisoning people at stadiums, setting off dirty bombs, blowing up nuclear power stations, blowing up embassies, bombing more office buildings, striking US military installations, spraying anthrax, etc., etc., etc.,.......
I am unaware of any particular reason why the "hijacking of planes for suicide attacks on US territory plan" should have been particularly scrutinized as opposed to the others, many of which were spoken of with far greater frequency..