Citing multiple failures across the government to detect and prevent the Sept. 11, 2001, terrorist plot, the bipartisan commission investigating the attacks today called for the appointment of a new high-level intelligence chief and the establishment of a national counterterrorism center to help overcome deep institutional failings and deal with the likelihood of another major terrorist assault.

"Since the [Sept. 11] plotters were flexible and resourceful, we cannot know whether any single step or series of steps would have defeated them," the 567-page report says in its executive summary. "What we can say with confidence is that none of the measures adopted by the U.S. government from 1998 to 2001 disturbed or even delayed the progress of the al Qaeda plot. Across the government, there were failures of imagination, policy, capabilities and management."

The report adds: "The most important failure was one of imagination. We do not believe leaders understood the gravity of the threat."

President Bush, presented with a copy of the report at the White House this morning, said he would study the panel's "very constructive recommendations." But he did not immediately commit his administration to any fundamental changes.

Sen. John F. Kerry of Massachusetts, the Democratic presidential challenger to Bush in the November election, called on the administration and Congress to move rapidly to implement the commission's recommendations, adding, "This is a time to come together."

If he is elected and adequate steps have not been taken, Kerry said, he would convene "an emergency security summit" of agency heads and leaders of both parties to make "the administrative changes necessary to protect this country."

In a news conference coinciding with the formal release of the report, commission members said urgent action is needed to implement the recommended reforms in the face of what they described as inevitable attempts by terrorists to inflict even greater casualties on the United States.

"It is not our purpose to assign blame," said the commission's chairman, Thomas H. Kean, the Republican former governor of New Jersey. "Our goal is to prevent future attacks. Every expert with whom we spoke told us an attack of even greater magnitude is possible, and even probable."

"We do not have the luxury of time," Kean said. "We must prepare, and we must act."

The vice chairman of the 10-member panel, Lee H. Hamilton, a former Democratic congressman from Indiana, cautioned that "there is no silver bullet or decisive blow that can defeat Islamist terrorism."

Outlining the commission's recommendations, Hamilton said a new National Counterterrorism Center would "unify all counterterrorism intelligence and operations across the foreign and domestic divide in one organization." He said, "Right now these efforts are too diffuse across the government."

Hamilton said a new national intelligence director would address the need for "a much stronger head of the intelligence community" in the United States.

"The intelligence community needs a shift in mindset and organization," Hamilton said, so that intelligence agencies operate under the principle of unity of command, "with information-sharing as the norm."

Hamilton said the Sept. 11 commission, which produced its report unanimously with no dissenting views, did not support the creation of a new domestic intelligence agency, such as Britain's MI5.

Instead, the report calls for reform within the FBI and the establishment of "a stronger national security work force" within the agency, Hamilton said.

In addition, the panel concluded that the United States needs to improve transitions from one administration to the next to ensure "that this nation does not lower its guard every four or eight years," Hamilton said.

The report also calls for strengthening the ability of the House and Senate intelligence committees to perform their oversight work.

Panel member James R. Thompson, a former Republican governor of Illinois, underscored the urgency of the commission's recommendations. If the panel's suggested reforms are rejected, "the Congress and the president need to tell us what's better," he said in the news conference.

"If something bad happens while these recommendations are sitting there, the American people will quickly fix responsibility for failure," he said.

Discussing the commission's findings in a CNN interview this morning, Sen. Pat Roberts (R-Kan.), the chairman of the Senate intelligence committee, said there are already several bills in Congress to create a "national intelligence director" of the sort recommended by the panel. As for the recommendation on creating a new national counterterrorism center, Roberts said that "basically, we ought to be able to do that, because that would be an expansion of what we call the threat information center in the homeland security agency."

While the report faults institutional failures, it stops short of blaming Bush or former president Bill Clinton for failing to prevent the Sept. 11 attacks.

Even so, Kean said in the news conference, "we do believe both presidents could have done more in this area." He said they both "were not served properly by the intelligence agencies of this country" and were not provided with the necessary information for the decisions they needed to make.

Said Hamilton, "They, like the rest of us, did not understand . . . that 3,000 people could be killed in an hour."

Earlier, Bush hailed the report's recommendations in an appearance in the White House Rose Garden as he stood between Kean and Hamilton.

The two have "done a really good job of learning about our country, learning about what went wrong prior to Sept. 11th, and making very solid, sound recommendations about how to move forward," Bush said. "I assured them that where government needs to act, we will."

He added: "They recognize what I recognize, and America recognizes, that there's still a threat and that we in government have an obligation to do everything in our power to safeguard the American people."

Bush said he looks forward to studying the report's recommendations and working with "responsible parties within my administration" and the Congress to move ahead on them.

"You did a wonderful job," he told Kean and Hamilton. Kean thanked Bush for what he called "unprecedented access to documents and cooperation from your administration."

The commission's final report was released after a 20-month investigation, a probe that Kean said involved the review of 2.5 million pages of documents and interviews with 1,200 individuals.

The report says that the danger from terrorist leader Osama bin Laden and his al Qaeda network "was not a major topic for policy debate among the public, the media, or in the Congress" before the attacks that killed more than 3,000 people. "Indeed, it barely came up during the 2000 presidential campaign."

Regarding the prospect of future terrorist attacks, the report says, "The problem is that al Qaeda represents an ideological movement, not a finite group of people. It initiates and inspires, even if it no longer directs. In this way it has transformed itself into a decentralized force. Bin Laden may be limited in his ability to organize major attacks from his hideouts. Yet killing or capturing him, while extremely important, would not end terror. His message of inspiration to a new generation of terrorists would continue."

The document adds: "The enemy is not Islam . . . but a perversion of Islam. The enemy goes beyond al Qaeda to include the radical ideological movement, inspired in part by al Qaeda, that has spawned other terrorist groups and violence. Thus our strategy must match our means to two ends: dismantling the al Qaeda network and, in the long term, prevailing over the ideology that contributes to Islamic terrorism."

In calling for creation of a National Counterterrorism Center, the commission's report says it envisions a civilian agency "that would borrow the joint, unified command concept adopted in the 1980s by the American military" and would combine "the joint intelligence function alongside the operations work." The new agency "would build on the existing Terrorist Threat Integration Center and would replace it and other terrorism 'fusion centers' within the government," the report says.

It says a new "national intelligence director" should be located in the Executive Office of the President and report directly to the president, although confirmation of the director by the Senate would be required. It says the director should have three deputies: for foreign, defense and homeland intelligence.

"Since long before 9/11 -- and continuing to this day -- the intelligence community is not organized well for joint intelligence work," the report says in explaining the rationale for the new director. "The community's head -- the Director of Central Intelligence -- has at least three jobs: running the CIA, coordinating a 15-agency confederation, and being the intelligence analyst-in-chief to the president. No one person can do all these things."