WASHINGTON - The September 11 commission has criticised both the Bush and Clinton administrations for failing to fully grasp or effectively combat the threat posed by al Qaeda.
It has recommended a shake-up of US intelligence to meet future dangers.
The final report issued unanimously by the 10-member commission pointed to "deep institutional failings" and missed opportunities to thwart the hijackings carried out by al Qaeda operatives, which killed almost 3000 people in 2001.
"Terrorism was not the overriding national security concern for the US government under either the Clinton or the pre-9/11 Bush administrations," the 567-page report said.
The commission recommended the appointment of a national intelligence director and creation of a national counter-terrorism centre to better co-ordinate and share information about future terrorist threats.
"The National Intelligence Director should oversee national intelligence centres to provide all-source analysis and plan intelligence operations for the whole government on major problems," the report said.
Numerous other recommendations included declassifying intelligence spending, upgrading the computer technology used by US intelligence and reorganising congressional oversight.
The bulk of the report consisted of a detailed narrative of the years and months leading up to the attacks. In the summer of 2001 -- the so-called summer of threat -- many officials feared that "something terrible" was planned, the report said.
In the face of intelligence warnings that were numerous but not specific, Attorney General John Ashcroft assumed the FBI was taking necessary action and never asked the agency what it was doing or gave it specific instructions.
"Domestic agencies never mobilised in response the threat. They did not have direction and did not have a plan to institute. The borders were not hardened. Transportation systems were not fortified ... The public was not warned," the report said.
CIA Director George Tenet stepped down this month under a barrage of complaints about intelligence failures before September 11 and before last year's US-led invasion of Iraq.
The report listed 10 missed opportunities by the CIA and the FBI, four in August 2001, to interpret or share information that could have helped them penetrate the September 11 plot.
As late as September 4, 2001, a week before the suicide hijacking attacks on the World Trade Centre and the Pentagon, the Bush administration had not decided whether Osama bin Laden's al Qaeda operation was a "big deal."
"The 9/11 attacks were a shock, but they should not have come as a surprise. Islamist extremists had given plenty of warning that they meant to kill Americans indiscriminately and in large numbers," the report said.
"The most important failure was one of imagination. We do not believe leaders understood the gravity of the threat," the report said. "The terrorist danger from bin Laden and al Qaeda was not a major topic for policy debate among the public, the media, or in the Congress. Indeed, it barely came up during the 2000 presidential campaign."
By criticising both the Bush and Clinton administrations, the five Republicans and five Democrats on the panel gave ammunition to both parties in the heated presidential race. But President George W Bush was seen as more vulnerable because he was in office when the attacks took place.
The White House, which initially tried to block the establishment of the commission, has been awaiting the report nervously, hoping it would not interfere with Bush's campaign for re-election in November.
Bush, reacting swiftly to the report, called it "solid and sound" but did not discuss its findings. He said he would study the recommendations, many of which he described as common sense and constructive.
The report concluded there was no collaboration between Iraq and al Qaeda, one of Bush' central arguments for launching an invasion of Iraq last year.
It did list numerous links between al Qaeda and Iran but said there was no evidence that the Iranians were aware of the planning for the attack.
Iranian officials facilitated travel of al Qaeda members, including several of the September 11 hijackers, through Iran on their way to and from their central bases in Afghanistan, and Iranian border officials were ordered not to stamp their passports.
The commission sharply criticised Congress for failing in its oversight role on terrorism and intelligence issues.
Former counterterrorism czar Richard Clarke, who served both former President Bill Clinton and Bush and who testified before the commission, said the report left many questions unanswered.
"The commission decided unanimity was more important than controversy. They did a very workman-like Washington report," Clarke said.
Overnight, US television networks broadcast newly released surveillance video from Washington's Dulles International Airport on the morning of the attacks that investigators view as one of the many missed opportunities.
The video shows five hijackers passing through security checkpoints. Four of them repeatedly set off alarms but were quickly cleared to board the flight that later crashed into the Pentagon. It was not clear what set the alarms off.