A commission probing the September 11 terrorist attacks was expected to conclude in a report out today that the US missed 10 opportunities to detect the deadly hijacking plot.
The Congressional cross-party panel is expected to say that six of the failings came under the leadership of the current President George Bush, and four under Bill Clinton.
Many of the opportunities would have been "long shots" it will concede, according to reports.
The 10-member National Commission on Terrorist Attacks Upon the US, reportedly stops short of saying the attacks were preventable.
America's most powerful politicians, security and intelligence officials were called to give evidence, often in public, during the 20-month probe.
Among them were Secretary of State, Colin Powell, Defence Secretary Donald Rumsfeld and the recently-retired CIA Director George Tenet.
All said they did not believe the attacks could have been prevented.
National Security Adviser Condoleezza Rice initially rejected appeals for her to give evidence in open session, but relented amid a storm of public protest.
Some hearings proved politically damaging for President George Bush.
His former counter-terror adviser Richard Clarke said the US had "failed" the victims of September 11.
He accused Mr Bush of not treating the terror threat as an "urgent" issue.
He said Mr Bush "ignored terrorism for months, when maybe we could have done something to stop 9-11".
President Bush met with the commission, accompanied by Vice-President Dick Cheney, in closed session.
Government officials and others familiar with the 600-page commission report told the Washington Post yesterday that it identifies 10 "operational opportunities" to detect or derail the hijacking plot.
One commissioner, who was not named, told the newspaper: "There clearly were many opportunities out there that were not taken advantage of.
"From that, some will conclude it could have been prevented, others will say it might have been prevented and the rest will say it's impossible to tell.
We said we couldn't get an answer to this."
Among the expected findings are:
:: The CIA missed significant "telltale indicators" of an impending terrorist attack. Among the indicators were a July 2001 FBI report of terrorist interest in aircraft training in Arizona, and the August 2001 arrest of terrorism suspect Zacarias Moussaoui because of his suspicious behaviour at a Minnesota flight school.
:: Some of the 19 hijackers were able to enter the US even though they lacked proper documentation. On the day of the attacks several were allowed through airport security after being stopped for suspicious behaviour.
Two of the hijackers were on a government terrorist watch list but were allowed to board flights because the Federal Aviation Administration (FAA) and airlines had not been told.
:: Under the administrations of both Mr Bush and his predecessor Bill Clinton, diplomacy was used rather than military action to combat the growing al Qaida threat. The US may have been able to kill al Qaida chief Osama bin Laden in the late 1990s, but the CIA said it never received a clear directive to do so.
:: On the day of the attacks, FAA officials were slow to alert the military to the hijackings, and some fighter pilots were never told why they were scrambled.
The commission is also expected to conclude that up to 10 of the September 11 hijackers travelled through Iran before the terror attacks.
President Bush said on Monday that the US was probing possible Iranian links to September 11.
US officials have stressed that it is not believed that Iran had direct involvement in September 11 but was apparently willing to allow al Qaida members to cross its borders unchecked.
After laying out its criticisms, the commission is set to recommend restructuring of America's intelligence and security apparatus, including the appointment of a Cabinet-level intelligence director who would oversee the CIA, the FBI and other agencies.
The report was being published at 1630BST in Washington, DC.Reuse content