At a two-day hearing this week, the federal commission investigating the attacks revealed US authorities had numerous opportunities to stop the hijackers, including many face-to-face encounters.

The missteps included miscommunications about al-Qaeda operatives dating back to the mid-1990s, hijackers who were allowed to repeatedly enter the United States even with false or the wrong visa papers, and missed chances to stop suspects at airport security checkpoints despite warning signs.

"We were asleep. Opportunities were lost," said former New Jersey Gov. Thomas Kean, a Republican who chairs the bipartisan National Commission on Terrorist Attacks Upon the United States. "The hijackers analysed our system and developed a plan they felt sure would beat it in every case, and 19 out of 19 succeeded."

Congress established the commission to study the nation's preparedness before Sept. 11, 2001, its response to the attacks, and to recommend ways to prevent such disasters.

The errors documented by the commission date back to just after the 1993 World Trade Center bombings and continued until the fateful day in 2001.