The US national commission examining the 11 September 2001 attacks has found no "credible evidence" that Iraq helped al-Qaeda militants carry them out.
The inquiry wants to know whether the attacks could have been prevented
The statement appears in a report on al-Qaeda published before the final public session of the commission.
It contradicts remarks by the US vice-president about Saddam Hussein's "long-established ties" with al-Qaeda.
Iraq's alleged links with al-Qaeda were part of the justification the Bush administration gave for invading Iraq.
The 11 September attacks killed nearly 3,000 people after members of Osama Bin Laden's al-Qaeda network flew three hijacked planes into New York's World Trade Center and the Pentagon, with a fourth crashing in Pennsylvania.
The commission, drawn from both Republicans and Democrats, published two separate preliminary reports: an overview of al-Qaeda and an outline of the 11 September plot.
It concludes that senior al-Qaeda suspect Khalid Sheikh Mohammed initially proposed a hijacking attack involving 10 planes to hit an expanded list of targets that would include the CIA and FBI headquarters, unidentified nuclear plants and tall buildings in California and Washington state.
The more ambitious plan was reportedly rejected by Bin Laden.
A final report on the commission's findings is due on 28 July.
But preliminary statements published by the commission on a range of issues are building up into a complex picture of missed opportunities and some of it does not make pleasant reading for the Bush administration, says BBC diplomatic correspondent Jonathan Marcus.
Bin Laden spurned
The report on al-Qaeda, entitled Overview of the Enemy, describes the roots of the militant network and its activities.
It says Osama Bin Laden had explored the possibility of co-operation with Iraq, despite his opposition to Saddam Hussein's secular regime.
A senior Iraqi intelligence officer had met Bin Laden in 1994 to hear his requests for space to establish training camps and assistance in procuring weapons - but Iraq had not responded.
"There have been reports that contacts between Iraq and al-Qaeda also occurred after Bin Laden had returned to Afghanistan, but they do not appear to have resulted in a collaborative relationship," the statement says.
"We have no credible evidence that Iraq and al-Qaeda co-operated on attacks against the United States."
The report on al-Qaeda also finds:
- Bin Laden did not fund al-Qaeda through a personal fortune - it relied on a fundraising network.
- There is no convincing evidence that any government financially supported al-Qaeda before the 11 September attacks.
- The 1998 attacks on US embassies in Kenya and Tanzania "were planned, directed, and executed by al-Qaeda, under the direct supervision of Bin Laden and his chief aides".
- Al-Qaeda is "far more decentralised", now that Bin Laden has lost his Afghan base.
More attacks likely
The commission's second report, entitled Outline of the 9/11 Plot, paints a picture of al-Qaeda members integrating themselves into Western societies before coming together to strike at America, and shows Bin Laden dominating the organisation's decision-making.
The commission finds that al-Qaeda is still "extremely interested in conducting chemical, biological, radiological, or nuclear attacks".
The point was reinforced by a CIA expert testifying on Wednesday.
"Al-Qaeda... has by no means been defeated and though weakened, it continues to patiently plan its next attacks," said the expert, identified as Dr K.
"They may strike next week, next month or next year but they will strike," he warned.
The expert is one of several law enforcement and intelligence experts on al-Qaeda testifying before the National Commission on Terrorist Attacks Upon the United States.
On Thursday, top military and civilian aviation officials - including General Richard Myers, Chairman of the Joint Chiefs of Staff - will testify about their agencies' responses to the attacks.