"But this isn't an unbiased source".
But it is also true that the US, in funneling so much money and perhaps other special assistance to pakistan we are not really doing any good for the either the region or for ourselves. Perhaps also we give money to Pakistan and let them empower the taliban in afghanistan because we made a promise to do so when we were fighting the russians.
The US will no doubt pay for aspects of this war by simply printing money and passing it out to Pakistan, Uzbekistan, Russia and no doubt some afghans. Look for inflation. What if we'd instead used some of those resources in advance to avoid the whole problem?
Rumsfeld and others believe in pre-empting bad things happening to us. So, without any criticism of our establishment, this letter documents that 911 could've been pre-empted.