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The Sept. 11, 2001, attacks (search) occurred largely because of a "failure of imagination" by the U.S. government to deal with a gathering threat that eventually would forever alter the way America deals with terrorists, the independent commission investigating the attacks concluded in its report released Thursday.

One of the most monumental recommendations put forth is the creation of a Cabinet-level national intelligence chief to better coordinate operations among the 15 government agencies that deal with intelligence.

[To read the full report, click here. To read an executive summary of the report, click here. Adobe Acrobat is required to read both pdf documents.]

"On Sept. 11, 2001, 19 men armed with knives, boxcutters, mace and pepper spray penetrated the defenses of the most powerful nation of the world. They inflicted unbearable trauma on our people and at the same time, they turned the international order upside down," Thomas Kean, co-chairman of the National Commission on Terrorist Attacks Upon the United States (search), said in releasing the report to the public.

"On that September day, we were unprepared. We did not grasp the magnitude of the threat that had been gathering over a considerable amount of time," he added. "This was a failure of policy, management, capability and, above all, a failure of imagination … the United States government was simply not active enough in combating the terrorist threat before 9/11."

But not one president — George W. Bush or Bill Clinton — was blamed for his actions or lack thereof more than another.

"They, like the rest of us, did not understand the gravity of the threat ... they did not think that 3,000 people could be killed in an hour's time," panel co-chairman Lee Hamilton said. "All of us had signals ... we simply did not put them together to understand that terrorism was the predominant national security threat to the United States."

Kean said there was only one mention of terrorism during the 2000 elections and added that the seriousness of the threat wasn't realized because intelligence information didn't reach the uppermost echelons of government.

"I can tell you that the two presidents of the United States were not well served by those agencies and they did not, in my opinion, have the information they needed to make the decisions they needed to make," he said.

'We Cannot Let Our Guard Down'

Calling the U.S. government's response to the attacks "improvised and ineffective," Kean said, "there's no single individual who was responsible for our failures, yet individuals and institutions cannot be absolved of responsibility."

"It is not our purpose to assign blame … we look back so that we can look forward," he added. "Our goal is to prevent future attacks."

Experts told the panel than an attack of even greater magnitude than that of Sept. 11 is not just possible but probable.

"We do not have the luxury of time — we must prepare and we must act," Kean said. "We cannot let our guard down."

Commissioner James Thompson said time cannot be wasted in implementing the commission's recommendations.

"Now we've been warned, specifically warned, and we've been told by everyone from the president of the United States on down it's going to happen again," Thompson said. "And if it happens and we don't move, the American people are entitled to make very fundamental judges about that."

Senate Intelligence Committee Chairman Pat Roberts, R-Kans., later said he's ready to ask commissioner to testify in hearings to put the recommendations into place, while

Hamilton said there's no "silver bullet or decisive blow" that can defeat terrorism.

"We need to play offense to kill or capture the terrorists, deny them sanctuaries and disrupt their ability to move money and people around the globe," he said.

President Bush was briefed on the report Thursday morning, when he thanked the other commission members, saying they "have left their mark in a very constructive and positive way."

"They've done a really good job at learning about our country, learning what went wrong prior to Sept. 11 and making very solid, sound recommendations about how to move forward," Bush said at the White House. "I assured them that where government needs to act, they will."

"There's still a threat, and we in government have an obligation to do everything in our power to safeguard the American people," Bush said, promising to move forward on the recommendations. "The most important duty we have is the security of our fellow countrymen."

Sen. John Kerry, the presumptive Democratic presidential nominee, got a 10-minute briefing on the 575-page report — culled from 2.5 million documents and 900 interviews — by phone Thursday.

"Nearly three years after terrorists attacked our shores and murdered our loved ones, this report carries a simple message about our current state of security for every American who remembers that dark September day: We can do better. We must do better. And it's time to act — now," Kerry said in a statement after the report was released.

The Iraq-Al Qaeda Connection

While the report did not say there was a direct link between Al Qaeda and Iraq regarding the Sept. 11 attacks, it did say there has been communication and some sort of a relationship between the terror group and Saddam Hussein in the past.

The report noted that Usama bin Laden began exploring a possible alliance with Iraq in the early 1990s and says that an Iraqi delegation traveled to Afghanistan in July 1998 to meet with the ruling Taliban and with bin Laden.

Intelligence indicates that Iraq may have offered bin Laden safe haven, but he declined after apparently deciding that Afghanistan was a better location.

And while there is strong evidence that Iran facilitated the transit of Al Qaeda members into and out of Afghanistan before the attacks, the report didn't say Tehran actually knew of the plot.

The report also recommends unifying and strengthening congressional oversight of intelligence activities, strengthening the FBI and homeland defenders and recruiting more covert spies in countries like Afghanistan, Pakistan and some parts of Yemen, where Al Qaeda is active and recruiting followers.

New Intelligence Chief

The proposed Cabinet-level national director of intelligence would have authority over the CIA, FBI and other agencies. The White House administration is reserving judgment on that recommendation, and officials doubt it could be approved by Congress this year.

"We think it's a very important recommendation to study," White House spokesman Dan Bartlett told FOX News, stopping short of endorsing the idea and saying the last thing Bush wanted to do was add another layer of bureaucracy to government. "That's something that we're very interested in and we're taking it seriously and we're going to work with the Congress on it."

The report did not, however, suggest setting up an American-style domestic spy agency within the FBI modeled after Britain's MI5. It recommends the creation of a giant National Counterterrorism Center, which would pull together intelligence from all agencies and help facilitate the sharing of sensitive information.

In addition, the report says the U.S. must establish a better means of processing and sharing the sensitive information it is privy to.

"Need to share must replace need to know," Hamilton said, adding that civil liberties protections must also be in place to "preserve the liberties that we are fighting for."

A more effective transition process between presidential administrations and national security officials must also be established, Hamilton said, "so that this nation does not lower its guard every four or eight years."

FOX News' Catherine Herridge contributed to this report.