The US commission investigating the 11 September attacks has blamed US leaders for failing to comprehend the gravity of the threat posed by al-Qaeda.
The report called on people "to remember how we all felt on 9/11"
Chairman Thomas Kean spoke of a failure of "policy, management, capability and, above all, imagination".
The measures adopted by the US from 1998 to 2001 did not disturb or even delay the plot, the final report says.
The commission recommends a wide-ranging overhaul of US intelligence services and congressional oversight.
"Where government needs to act, we will," President George W Bush said after receiving his copy of the report which comes after two years of exhaustive investigation.
Intelligence bodies have come under harsh criticism for failing to avert the airliner hijackings, in which about 3,000 people died.
An airport surveillance video of some of the hijackers has added to dismay over the ease with which planes were seized.
But no single individual is to blame, said Mr Kean, launching the report signed by all its members.
"Yet individuals and institutions cannot be absolved of responsibility," he said.
Bob Hughes, the father of a victim of the attacks, said "there is plenty of blame to go around".
"There's really no closure for me and my wife," Mr Hughes said.
Among the report's recommendations are:
BBC News Online world affairs correspondent Paul Reynolds says the measures proposed in the report amount to the most significant change since the CIA itself was founded after World War II.
- the creation of a national counter-terrorism centre "unifying strategic intelligence and operational planning against Islamist terrorists across the foreign-domestic divide"
- the establishment of a new national intelligence director to unify the intelligence community
- creating a "network-based information sharing system that transcends traditional governmental boundaries"
- strengthening congressional oversight
The bipartisan National Commission on Terrorist Attacks Upon the United States published the final report - almost 600 pages long - following testimony from more than 1,000 witnesses and examination of as many classified documents.
"The 9/11 attacks were a shock, but they should not have come as a surprise," the 10-member panel said.
The report said "Islamist extremists had given plenty of warning that they meant to kill Americans indiscriminately and in large numbers".
But "terrorism was not the overriding national security concern for the US government under either the [former President Bill] Clinton or pre-9/11 Bush administration," it says.
"The United States government was simply not active enough in combating the terrorist threat before 9/11," Mr Kean said.
He added that it had been impossible to determine whether the 11 September attacks could have been prevented - it was "possible", but this could not be borne out by facts.
"An attack of even greater magnitude is now possible and even probable. We do not have the luxury of time. We must prepare and we must act," Mr Kean said.
The report carries political weight and as such it cannot be ignored, says the BBC's Justin Webb in Washington.
Mr Bush's challenger in the November elections, John Kerry, has already said that if elected, he will immediately convene an emergency bipartisan security summit bringing to map out and implement a reform programme.