The commission investigating the attacks heard audiotapes of the hijackers as they committed their crimes.
The timeline of that day, carefully pieced together by the commission, shows a panicked and chaotic response to the hijackings, an improvised reflex, that had it been different, might have saved lives.
"And the FAA were unprepared for the type of attacks launched against the U.S. They struggled, never trained to meet, etc. In fact, the FAA did not notify military officials that American flight 11, which later crashed into the World Trade Center, had been hijacked," said Philip Zelikow of Norad.
Air traffic controllers only realized what was happening when they received a transmission from the plane.
"Nobody move. Everything will be okay. Just stay quiet," said hijacker Mohammad Atta.
Air Force fighters were eventually scrambled to deal with the developing threat, but military coordinators did not know in which direction to send the fighter jets, and it wasn't until after United Flight 175 hit the south tower that Norad air defenders were even aware of a second aircraft.
Further, American Flight 77, which would eventually crash into the Pentagon, traveled under the control of the hijackers for 36 minutes before impact.
Still, those testifying Thursday said there have been considerable improvements in the system.
"Because of the fixes, the remedies put in place, we would be able to shoot down all three, all four aircraft," said Gen. Eberhart.
President Bush, meanwhile, is disputing at least one of the commission's early findings. It rejects the commission's assertion that there was no link between Al Qaeda and Iraq.
"This administration never said that the 9/11 attacks were orchestrated between Saddam and Al Qaeda. We did say there were numerous contacts between Saddam Hussein and Al Qaeda," said the president.