Many experts have speculated about this; but as with all such 'classified' CIA matters getting proof is hard to come by. But listen to this BBC report from some months ago outlining in depth the firm and detailed American plans to attack Afghanistan and change the regime in Kabul weeks before 9/11. "Absolutely astounding" says the BBC reporter!
http://news.bbc.co.uk/olmedia/1550000/audio/_1550366_afghan01_arney.ram