This article gives the details. Apparently, a Presidential Daily Briefing from 21 September 2001, the details of which are still being kept secret, indicated that Iraq had no connection to the 9/11 attacks. The very existence of this document was kept secret from the Congress.
Bush KNEW that Iraq hadn't been involved, and yet he invaded in part because he claimed that in so doing he was fighting terrorism. The Administration's paid liars hinted and implied REPEATEDLY that Saddam was implicated in the attack on America. Many Americans still believe this, due in large part to Republican lies.
An outrage, pure an simple.