Fairly certainly Muslim extremists actually carried out the attack, but the Israelis almost certainly knew an attack was imminent, and probably knew the exact circumstances of it, hence the dancing Israelis.

Neocons in the Bush administration also almost certainly knew a major attack of some sort was imminent and intentionally ignored Al Queda and these warning signs in order to let it happen. Whether some of them knew specific details is doubtful, but who knows. Id love to know if any of the prominent neocons were at the pentagon at the time of the attack on it.