US: While the US administration has recently been criticised for overstating evidence of contacts between al-Qaeda and Iraq, it now turns out there was much more substantial co-operation between the Afghanistan-based terrorist group and the government of Iran.
Between eight and 10 of the 9/11 hijackers passed through Iran in the period from October 2000 to February 2001, and from October 2000 Iranian border guards were instructed not to hinder or harass al-Qaeda members as they exited and entered Afghanistan across the Iranian border.
This new information is contained in the final report of the bi-partisan commission investigating the attacks on the US which will be issued on Thursday, according to Time magazine, which cites senior US officials.
The Iranian connection, which could lead to a new crisis in Washington-Tehran relations, highlights a major drawback in US President George Bush's policy of pre-emptive strikes.
Because of the war against Iraq, which has overstretched the US military, America's options for dealing with Iran's complicity in terrorist attacks have been considerably narrowed.
The new intelligence was reportedly gleaned from al-Qaeda detainees, including captured Yemeni al-Qaeda operative Waleed Mohammed bin Attash who organised the October 2000 attack on the USS Cole, and from up to 100 separate electronic intelligence intercepts at the National Security Agency.
It was presented to the 10-member commission as it neared the end of its 16-month investigation into the source and execution of the 9/11 attacks.
The commission does not apparently provide evidence that the Iranian government knew of actual plans to hijack planes for use as missiles in the US.
It does say, however, that Iran offered to assist al-Qaeda after the attack on the USS Cole in further operations against the US, but that Osama bin Laden declined because he did not want to alienate his supporters in Saudi Arabia.
While making the case that Iran was a threat to America interests through its co-operation with al-Qaeda, the report concludes that al-Qaeda and Iraq did not form a working relationship, despite claims by top Bush administration officials, especially the Vice President, Mr Dick Cheney, of links between al-Qaeda and Saddam Hussein.
The 8-10 al-Qaeda activists who passed through Iran were reportedly "muscle" hijackers, recruited to take control of the four hijacked aircraft and subdue the crew and passengers.
The 9/11 commission's chairman, Mr Thomas Kean, hinted at the new evidence last month when he said he believed "there were a lot more active contacts, frankly, with Iran and with Pakistan than there were with Iraq."
The report recommends that the US create a powerful national counter-terrorism centre and a new cabinet-level director to oversee the CIA, FBI, and other intelligence agencies connected with the national security council and the Pentagon, according to the New York Times.
The former CIA director Mr George Tenet announced his resignation this summer just before the Senate Intelligence Committee found that pre-war intelligence on weapons of mass destruction presented by the CIA to the Bush administration was wrong.
However, the 9/11 commission proposal for fundamental reform will meet resistance from the CIA.
Acting CIA chief Mr John McLaughlin argued on Fox News Sunday that only "modest changes" were needed to give the head of the CIA the power to carry out the function of a new intelligence tsar "well and appropriately".
The commission's final report will run to 600 pages and will highlight the intelligence failures that allowed the 19 hijackers to enter the US and carry out attacks on the World Trade Centre and the Pentagon that killed almost 3,000 people.
Mr Kean, a Republican, and the commission's Democratic vice chairman, Mr Lee Hamilton, have both said that the evidence gathered by their staff showed the attacks could probably have been prevented by better intelligence co-operation, and by proper congressional and administration oversight.