The final report of the "9/11 Commission", scheduled to be released Thursday, is expected to list as many as 10 missed opportunities to deter or derail the Sept. 11, 2001 hijackings.
News accounts say the nearly 600-page report includes a list of 10 "operational opportunities" the Bush and Clinton administrations missed to potentially unravel the 9/11 plot.
But the document, according to news reports, stops short of saying the attacks, which killed about 3000 people, could have been prevented.
The top Democrat in the House of Representatives, Congresswoman Nancy Pelosi, spoke to reporters after being briefed on the report by commission leaders.
"It is not about assigning blame," she said. "It is about preventing any future acts of terrorism to our country." The missed opportunities cited in the report include the Central Intelligence Agency's (CIA) failure to add two of the 19 hijackers' names to a terrorism watch list, the Federal Bureau of Investigation's (FBI) handling of the August 2001 arrest of accused September 11 conspirator Zacarias Moussaoui, and several failed attempts to kill or capture al-Qaida leader Osama bin Laden.
News accounts say the report calls many of the missed opportunities long shots, and that others would have required a lucky sequence of events to change the outcome.
The commission report also finds that al-Qaida had a stronger relationship with Iran and the Islamic extremist group, Hezbollah, than it did with the regime of Iraqi leader Saddam Hussein.
News reports earlier this week said that as many as 10 of the 9/11 hijackers had transited through Iran before the attacks.
The State Department includes Hezbollah among a list of terrorist groups, and says Iran is a state sponsor of terrorism.
The commission report is expected to propose a number of reforms to the intelligence community, including the creation of a new Cabinet-level position to oversee all U.S. intelligence operations.