The independent commission investigating the 2001 terrorist attacks is proposing a sweeping overhaul of U.S. intelligence gathering in its final report to Congress and the Bush administration.

The report totals more than 500 pages and comes after 20 months of exhaustive investigation by the bipartisan, 10-member commission into what went wrong on September 11th and how to prevent future attacks.

Commission Chairman Tom Kean told a Washington news conference that the most important failure related to the 9/11 attacks was one of imagination, that leaders in both the Clinton and Bush administrations never fully realized the threat al-Qaida posed to the U.S. homeland.

"And since the plotters were flexible and resourceful, we cannot know whether any single step or series of steps would have defeated them," he said. "What we can say with a good deal of confidence is that none of the measures adopted by the United States government before 9/11 disturbed or even delayed the progress of the al-Qaida plot."

The report pointed to what it called "deep institutional failings" in the government by missing several opportunities to uncover the plot. However, the commission does not say that the attacks were preventable.

The 9/11 commission recommends a major overhaul of U.S. intelligence functions to help prevent future attacks. The main proposal is for the appointment of a national intelligence director who will report directly to the president and the creation of a national counterterrorism center to better coordinate information from the CIA and the other intelligence agencies in the government.

The recommendations also include improved immigration screening to keep terrorists out of the country and a more focused foreign policy approach to reach out to moderate Muslims around the world.

"We need to join the battle of ideas within the Islamic world, communicating hope instead of despair, progress in place of persecution, life instead of death," said former Congressman Lee Hamilton, the 9/11 commission's vice chairman. "This message should be matched by policies that encourage and support the majority of Muslims who share these goals."

Prior to its public release, the report was presented to President Bush at the White House.

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

Democratic presidential contender John Kerry was briefed on the report and he urged the president and Congress to act quickly on the commission's recommendations.

Some congressional leaders have suggested that enacting some of the changes could take months. Congressman Robert Menendez, a Democrat from New Jersey, says that would be unfortunate.

"I think the report gives us an opportunity to understand the sense of urgency of responding to some of the recommendations and the findings because I think, to some degree, we may have lost that sense of urgency and I think the report will be a catalyst for that," he said.

Among the commission's recommendations is better oversight by Congress.

The report sparked a mixed reaction among the families of victims of the 9/11 attacks, many of whom pushed for the creation of the commission in the first place.

"Unfortunately, what we expect they will get in the end is a whitewash and what most Americans should greet as a farce, an out and out cover up and a shameful, colossal spin-job [deception]," said Kyle Hence, the co-founder of a group called 9/11 Citizens Watch.

Commission Chairman Tom Kean says the report should not be seen as a divisive political document but as a call to arms to ward off future terrorist threats.

"We face a determined enemy who sees this as a war of attrition, indeed as an epochal struggle," he said. "We expect further attacks. Against such an enemy, there can be no complacency. This is the challenge of our generation. As Americans, we must step forward and we must meet that challenge."

The 9/11 report also sheds new light on the relationship between al-Qaida and Iraq. The report says that although there were friendly contacts, it did not evolve into what it called a "collaborative relationship." The report also details links between al-Qaida and Iran, but says there is no evidence that Iran was involved or was aware of the 9/11 attacks.