US air officials had warnings of al-Qaida threat:
http://www.guardian.co.uk/world/2005/feb/11/september11.alqaida
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...It reveals that, in spring 2001, the FAA had warned US airports that hijackers intending to commit suicide "in a spectacular explosion" would be most likely to select domestic targets. The FAA is criticised for being more concerned with reducing airline congestion, lessening delays, and easing airlines' financial problems than with deterring a terrorist attack.
The report also accuses the authority of failing to pursue domestic security measures that could have helped counter the threats that led to September 11. Such actions could have included the toughening of airport screening procedures for weapons or expanding the use of on-flight air marshals, it said.
FAA spokeswoman Laura Brown said the agency had been well aware of the threat posed by terrorists before the attacks, and had taken substantive steps to counter it. "We had a lot of information about threats," she said. "But we didn't have specific information about means or methods that would have enabled us to tailor any counter measures...
The 9-11 Commission Report:
http://www.archives.gov/research/9-11-commission/index.html
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...Before September 11, the airport security system was run by the airlines but overseen by the FAA. After the attacks, the government ordered cockpit doors to be strengthened, took over screening of passengers and bags at airports, and coordinated "watch lists" of known or suspected terrorists among intelligence agencies.
Like previous commission documents, the report did not find evidence that the government had specific warning of the September 11 attacks...