The nation's law enforcement and intelligence agencies did not discover the plot. Airport security screeners did not find the hijackers' weapons.

But could military jet fighters, the final line of defense, have stopped or lessened the destruction on Sept. 11, 2001, by shooting down airliners aimed at some of the nation's best-known buildings?

On Thursday, the Sept. 11 commission will end its series of public hearings by taking up that question. They will examine the performance on that day of the Federal Aviation Administration (search), which manages the nation's air traffic; and NORAD, the North American Aerospace Defense Command (search), which defends U.S. airspace.

Kristen Breitweiser of Monmouth, N.J., whose husband, Ronald, died in the World Trade Center (search), said a lack of foresight on the part of those agencies was compounded by officials' mistakes on the morning of Sept. 11.

"I think we were ill-prepared, and I think people showed poor judgment," Breitweiser said. The plane that crashed into the Pentagon, in particular, could have been stopped, she contends.

Both NORAD and FAA officials respond to the criticism by describing how they've changed since Sept. 11. They have established chains of communication. Generals have been given authority to order the fighter pilots to shoot down hijacked aircraft. The number of warplanes on alert has been increased, and fighters are put on patrol over U.S. cities and events deemed possible terrorist targets.

When the Sept. 11 terrorists struck, the United States and Canada were defended by 20 fighter aircraft, arrayed in pairs in 10 locations, said Lt. Col. Roberto Garza, a NORAD spokesman. They were kept armed and fueled, with pilots nearby, ready to take off in less than 15 minutes.

The fighter defenses were a remnant of the Cold War, when North America worried more about intercepting Soviet bombers attacking from across the Arctic Circle. Of those pairs, six were on the East Coast, a NORAD spokesman said: two in Massachusetts, two in Virginia and two in Florida. The others were in Canada, Alaska, the West Coast and Texas.

But their focus was directed outward, toward threats that might approach American coastlines. The Florida fighters, for example, had their eyes on Cuba, which maintains an air force of MiG fighters.

Pre-Sept. 11, the view was that terrorist hijackings were essentially political acts, not necessarily destructive. Hijackers were expected to order the plane to fly to a destination, or land it and negotiate for the release of their hostages. Hijackings over the United States were a problem for law enforcement, not a military one.

The best information about the sequence of events on Sept. 11 comes from a timeline provided by NORAD in the months after the attacks. NORAD spokesman Garza said some aspects of the timeline now are considered inaccurate, however, but he refused to specify which ones.

The two Boston flights that hit the World Trade Center, American Airlines Flight 11 and United Airlines Flight 175, were the first to take off and the first to be hijacked. According to the NORAD timeline, the FAA informed NORAD of the hijacks at 8:40 a.m. and 8:43 a.m., respectively. Flight 11 hit the North Tower a few minutes later.

A 8:52 a.m., two F-15 Eagle fighters (search) rocketed into the sky from Otis Air National Guard Base, Mass., too late and too far away to have any hope of reaching Flight 175, which struck the South Tower 10 minutes later.

Around this time, at 8:55 a.m., American Airlines Flight 77, aloft from Washington, broke from its flight path. The hijackers turned off the plane's transponder (search), and it stopped sending data to air traffic controllers. The FAA lost radar contact with the plane.

It is at this point that relatives of the family members say the FAA and NORAD could have responded differently. They had 45 minutes until the plane would hit the Pentagon.

By then, American skies were in chaos. At one point, the FAA was tracking 11 planes that it feared could have been hijacked, said Laura Brown, a spokeswoman for the agency. Air Force fighters were taking off from bases unarmed, and someone floated the idea of using one of them to ram a hijacked airliner.

Still, two events would have been required for the Pentagon strike to have been averted.  

First, President Bush would have had to have ordered that any hijacked airliners be shot down; the military's rules of engagement (search) did not allow for that without such presidential intercession. Bush ultimately did make that call, but only after the Pentagon was hit.

Second, NORAD's F-16 Fighting Falcons (search) at Langley Air Force Base, near Norfolk, Va., would have had to have been launched sooner.

Why they weren't is unclear.

According to NORAD's timeline, those fighters received the scramble order at 9:24 a.m., 30 minutes after Flight 77 made an unauthorized turn. That suggests the FAA took an inexplicably long time to alert NORAD. But Jane Garvey, former head of the Federal Aviation Administration, said it had informed NORAD earlier in a telephone call.

Retired Maj. Gen. Larry K. Arnold, who was in charge of domestic air defenses on the day of the attacks, said it was "physically possible" that fighter jets could have beaten the civilian airliner to the Pentagon had they been activated earlier.

Another decision not made: NORAD did not launch all available fighters even when it became apparent that multiple suicide hijackings had taken place.

"It's inexplicable why they did not get air protection up in time to thwart that crash," Breitweiser said. She also asked why the F-15s sent to New York were not sent after Flight 77.

The three nearest fighters — an extra armed plane happened to have been ready at Langley — took off at 9:30 a.m. At 9:40 a.m., when Flight 77 hit the Pentagon, they were still 100 miles away.

Had they made it, the pilots and their commanders would have to live with the consequences of shooting down a plane filled with mostly innocent people. Some military officials have suggested they probably would be second-guessed for not allowing the passengers more time to try to take the plane back.

In addition, shooting down an airliner over a populated area has the chance of creating even more destruction. The fighter's missiles most likely would have created large pieces of wreckage, or created a single flaming, out-of-control missile careening into the city. The 124 lives lost at the Pentagon would have been spared, but what if the wreckage had hit a nearby apartment tower?

By 10 a.m., the FAA had ordered every nonmilitary plane in the skies to land. NORAD was tracking United Airlines Flight 93, the fourth hijacked plane, and Bush had given permission to shoot down any more hijacked airliners. Fighters over Washington circled, waiting to see where the plane would go.

At 10:03 a.m., it crashed into a field in Pennsylvania after its passengers tried to retake the aircraft.