The Sept. 11 commission reported yesterday that it has found no "collaborative relationship" between Iraq and al Qaeda, challenging one of the Bush administration's main justifications for the war in Iraq.

Along with the contention that Saddam Hussein was stockpiling weapons of mass destruction, President Bush, Vice President Cheney and other top administration officials have often asserted that there were extensive ties between Hussein's government and Osama bin Laden's terrorist network; earlier this year, Cheney said evidence of a link was "overwhelming."

But the report of the commission's staff, based on its access to all relevant classified information, said that there had been contacts between Iraq and al Qaeda but no cooperation. In yesterday's hearing of the panel, formally known as the National Commission on Terrorist Attacks Upon the United States, a senior FBI official and a senior CIA analyst concurred with the finding.

The staff report said that bin Laden "explored possible cooperation with Iraq" while in Sudan through 1996, but that "Iraq apparently never responded" to a bin Laden request for help in 1994. The commission cited reports of contacts between Iraq and al Qaeda after bin Laden went to Afghanistan in 1996, adding, "but they do not appear to have resulted in a collaborative relationship. Two senior bin Laden associates have adamantly denied that any ties existed between al Qaeda and Iraq. We have no credible evidence that Iraq and al Qaeda cooperated on attacks against the United States."

The finding challenges a belief held by large numbers of Americans about al Qaeda's ties to Hussein. According to a Harris poll in late April, a plurality of Americans, 49 percent to 36 percent, believe "clear evidence that Iraq was supporting al Qaeda has been found."

As recently as Monday, Cheney said in a speech that Hussein "had long-established ties with al Qaeda." Bush, asked on Tuesday to verify or qualify that claim, defended it by pointing to Abu Musab Zarqawi, who has taken credit for a wave of attacks in Iraq.

Bush's Democratic challenger, Sen. John F. Kerry (Mass.), sought to profit from the commission's finding. "The administration misled America, and the administration reached too far," Kerry told Michigan Public Radio. "I believe that the 9/11 report, the early evidence, is that they're going to indicate that we didn't have the kind of terrorists links that this administration was asserting. I think that's a very, very serious finding."

A Bush campaign spokesman countered that Kerry himself has said Hussein "supported and harbored terrorist groups." And Cheney's spokesman pointed to a 2002 letter written by CIA Director George J. Tenet stating that "we have solid reporting of senior level contacts between Iraq and al Qaeda going back a decade" and "credible information indicates that Iraq and al Qaeda have discussed safe haven and reciprocal non-aggression." Cheney's office also pointed to a 2003 Tenet statement calling Zarqawi "a senior al Qaeda terrorist associate."

White House spokesman Scott McClellan said the commission finding of long-standing high-level contacts between al Qaeda and Iraq justified the administration's earlier assertions. "We stand behind what was said publicly," he said.

Bush, speaking to troops in Tampa yesterday, did not mention an Iraq-al Qaeda link, saying only that Iraq "sheltered terrorist groups." That was a significantly milder version of the allegations administration officials have made since shortly after the Sept. 11, 2001, attacks.

In late 2001, Cheney said it was "pretty well confirmed" that Sept. 11 mastermind Mohamed Atta met with a senior Iraqi intelligence official before the attacks, in April 2000 in Prague; Cheney later said the meeting could not be proved or disproved.

Bush, in his speech aboard an aircraft carrier on May 1, 2003, asserted: "The liberation of Iraq is a crucial advance in the campaign against terror. We've removed an ally of al Qaeda and cut off a source of terrorist funding."

In September, Cheney said on NBC's "Meet the Press": "If we're successful in Iraq . . . then we will have struck a major blow right at the heart of the base, if you will, the geographic base of the terrorists who had us under assault now for many years, but most especially on 9/11."

Speaking about Iraq's alleged links to al Qaeda and the Sept. 11 attacks, Cheney connected Iraq to the 1993 World Trade Center bombing by saying that newly found Iraqi intelligence files in Baghdad showed that a participant in the bombing returned to Iraq and "probably also received financing from the Iraqi government as well as safe haven." He added: "The Iraqi government or the Iraqi intelligence service had a relationship with al Qaeda that stretched back through most of the decade of the '90s."

Shortly after Cheney asserted these links, Bush contradicted him, saying: "We've had no evidence that Saddam Hussein was involved with the September 11th." But Bush added: "There's no question that Saddam Hussein had al Qaeda ties."

In January, Cheney repeated his view that Iraq was tied to al Qaeda, saying that "there's overwhelming evidence" of an Iraq-al Qaeda connection. He said he was "very confident there was an established relationship there."

The commission staff, in yesterday's report, said that while bin Laden was in Sudan between 1991 and 1996, a senior Iraqi intelligence officer made three visits to Sudan, and that he had a meeting with bin Laden in 1994. Bin Laden was reported to have sought training camps and assistance in getting weapons, "but Iraq never responded," the staff said. The report said that bin Laden "at one time sponsored anti-Saddam Islamists in Iraqi Kurdistan."

As for the Atta meeting in Prague mentioned by Cheney, the commission staff concluded: "We do not believe that such a meeting occurred." It cited FBI photographic and telephone evidence, along with Czech and U.S. investigations, as well as reports from detainees, including the Iraqi official with whom Atta was alleged to have met. On the 1993 trade center bombing, the staff found "substantial uncertainty" about whether bin Laden and al Qaeda were involved.

At yesterday's hearing, commissioner Fred F. Fielding questioned the staff's finding of no apparent cooperation between bin Laden and Hussein. He pointed to a sentence in the first sealed indictment in 2001 of the al Qaeda members accused of the bombings of U.S. embassies in Kenya and Tanzania; that sentence said al Qaeda reached an understanding with Iraq that they would not work against each other and would cooperate on acquiring arms.

Patrick J. Fitzgerald, now a U.S. attorney in Illinois, who oversaw the African bombing case, told the commission that reference was dropped in a superceding indictment because investigators could not confirm al Qaeda's relationship with Iraq as they had done with its ties to Iran, Sudan and Hezbollah. The original material came from an al Qaeda defector who told prosecutors that what he had heard was secondhand.

The staff report on Iraq was brief. Though not confirming any Iraqi collaboration with al Qaeda, it did not specifically address two of the other pieces of evidence the administration has offered to link Iraq to al Qaeda: Zarqawi's Tawhid organization and the Ansar al-Islam group.

In October 2002, Bush described Zarqawi, a Palestinian born in Jordan, as "one very senior al Qaeda leader who received medical treatment in Baghdad this year, and who has been associated with planning for chemical and biological attacks."

Zarqawi wrote a January 2003 letter to bin Laden's lieutenants, intercepted at the Iraqi border, saying that if al Qaeda adopted his approach in Iraq, he would swear "fealty to you [bin Laden] publicly and in the news media."

In March, in a statement to the Senate Armed Services Committee, Tenet described Zarqawi's network as among groups having "links" to al Qaeda but with its own "autonomous leadership . . . own targets [and] they plan their own attacks."

Although Zarqawi may have cooperated with al Qaeda in the past, officials said it is increasingly clear that he has been operating independently of bin Laden's group and has his own network of operatives.

The other group, Ansar al-Islam, began in 2001 among Kurdish Sunni Islamic fundamentalists in northern Iraq, fighting against the two secular Kurdish groups that operated under the protection of the United States. At one point, bin Laden supported Ansar, as did Zarqawi, who is believed to have visited their area more than once. Tenet referred to Ansar as one of the Sunni groups that had benefited from al Qaeda links.

Vice President Cheney said in a speech on Monday that Saddam Hussein "had long-established ties with al Qaeda." The Sept. 11 panel said in a report that it has found no "collaborative relationship" between Iraq and al Qaeda.