For all that has been written about Sept. 11, one largely unexamined issue must be confronted if we're going to be more secure in the future.

Simply put: How could a handful of young Arabs outsmart the combined forces of the CIA, FBI, and other Western intelligence agencies? Did the hijackers make any mistakes that should have tipped us off? Were there moments when our side could have and should have spotted these hijackers before they struck?

The answer is yes.

The terrorists certainly succeeded by cunning, deception, and determination, and by taking advantage of our open society. But as you see in FRONTLINE'S "Inside the Terror Network," there were also times when alarm bells should have gone off.

• The failure of the INS to stop the attack's ringleader, Mohamed Atta, from entering the U.S. three times on a tourist visa in 2001, even though officials knew the visa had expired in 2000 and Atta had violated its terms by taking flight lessons.
• The CIA's failure to put another of the hijackers, Khalid al-Midhar, who had been filmed at a meeting to plot the bombing of the USS Cole in October 2000, on a terrorist watch list until August 2001, after he had already entered the United States to prepare for Sept. 11.
• The mystery of why U.S. authorities blocked the entry of a would-be hijacker pilot from Yemen but then failed to investigate his buddies, future hijackers Ziad Jarrah and Mohamed Atta, who had openly worked to get their Yemeni friend into a U.S. flight school.
• The FAA's failure to investigate hijackers Mohamed Atta and Marwan al-Shehhi after they abandoned a small plane on a busy taxiway at Miami International Airport in December 2000.
• The failure of the FBI in August 2001 to aggressively pursue a warning from a U.S. flight school that a very poor student pilot, Zacarias Moussaoui, had a peculiar interest in learning how to fly a 747 -- underlined by the pointed warning that a hijacker piloting a fully fueled airliner could use it as a bomb against ground targets.

There were other missed opportunities abroad. In Hamburg, German intelligence put a roommate of Mohamed Atta under surveillance in 1998-99, but they didn't realize that they had stumbled into Al Qaeda's Hamburg cell. At that time, Atta, the cell's leader, was gathering his hijack team and hatching the plot. But the Germans saw nothing suspicious and dropped their investigation.

Probably the greatest intelligence failure of all was a failure of imagination, a failure to conceive that Al Qaeda could recruit people smart enough to carry out such a deadly and sophisticated attack in the United States -- in short, a fundamental failure to understand the enemy well enough to gear up the necessary defenses.

"We're very good at listening on satellites or taking pictures from the sky of the former Soviet Union," asserts Sen. Bob Graham of Florida, chairman of the Senate Select Committee on Intelligence. "Where we have fallen down is the ability to deal with these much smaller, more flexible, and more violent terrorist groups that are now our enemy. ... There were serious breakdowns. ... We'd not been attacked in our homeland since Pearl Harbor. We did not feel that America itself was a target of terrorist activities. We were somewhat lax in that regard."

"We had lots of scenarios about what terrorists might try to do," former Assistant FBI Director Lew Schiliro told me, "But not that -- not what happened Sept. 11."

That's surprising because of a confession wrung out of an Al Qaeda terrorist named Abdul Hakim Murad, arrested in 1995 by Philippine investigators. Murad, a pilot trained in the U.S., confessed he had a plan for dive-bombing a plane into CIA headquarters. That report was passed on to U.S. authorities, but the FBI and CIA discounted this as the hare-brained scheme of a single terrorist.

It's also surprising because, as Lew Schiliro reports, the FBI and CIA had noticed a trend of steadily escalating and increasingly sophisticated Al Qaeda attacks against American targets, dating from the truck-bombing of the World Trade Towers in 1993. Osama bin Laden openly declared war on the United States -- on civilian as well as military targets.

In the mid 1990s, the FBI blocked a plot to bomb the World Trade Center and other economic targets in New York. But in 1998, Al Qaeda pulled off the bombing of two U.S. embassies in East Africa, and later, an attack on the American destroyer USS Cole in Yemen. In 2000, an Al Qaeda-backed terrorist, Ahmed Ressam, was caught sneaking into America, planning to bomb the Los Angeles airport.

It's also surprising because American intelligence knew that Al Qaeda had its sights on destroying the World Trade towers, as a symbol of America's wealth and economic might. In fact, on Sept. 11, Bob Blitzer, longtime chief of FBI counterterrorism until 1998, immediately understood the connection to two earlier Al Qaeda attacks.

"When I saw those planes hit the World Trade towers, I was not surprised," Blitzer told me. "The first thought on my mind was, 'My God, they finished the job.' I knew it was these guys because they are so committed. They have so much hatred of the West. Incredible determination. Incredible people."

But broadly speaking, Al Qaeda's determination and bin Laden's obsession with the World Trade towers were underestimated by U.S. intelligence. After the attack, I kept hearing from U.S. terrorism specialists that prior to Sept. 11, they were convinced that terrorism could not be "exported" over long distances and long periods of time.

In short, our experts' mindset was that terrorist cells had to operate close to their bases -- as in the case of Palestinian suicide bombers, recruited, trained and kept motivated right at home. Or the bombers who attacked the U.S. embassies in East Africa, operating out of nearby Somalia. Or the Yemeni terrorists who attacked the USS Cole, encamped near the harbor in Aden.

By that mistaken theory, the U.S. was safe.

In that mindset, our intelligence experts were predisposed to look for bin Laden to strike once again overseas. They did not imagine that Al Qaeda could recruit, train, and send to the U.S. a handful of well-educated Arabs in their 20s and 30s for 15 months, who would learn to fly small planes, get familiar with jet airliners, scout the actual flights they were going to hijack, slip in and out of the U.S., and use the Internet to communicate with each other and Al Qaeda, without our detecting them or their defecting.

That failure of imagination may have persuaded the FBI there was no need to comb the rosters of U.S. flight schools for potential hijackers. It may have made high FBI officials in August 2001 less keen to push to the hilt an investigation of a tip from the Pan Am Flight School in Minnesota that Zacarias Moussaoui had the earmarks of a potential hijacker planning to ram a fully fueled jet airliner into a big building.

The FBI arrested Moussaoui but did not aggressively probe his motives or connections, even though French intelligence supplied Washington a dossier connecting Moussaoui to known terrorist groups -- before Sept. 11.

A similar failure to understand the world we live in led the immigration service to take an over-simplified view of our friends and foes in the Middle East. It kept a wary eye on young men from Iraq, Iran, Syria, Algeria, and Yemen, but was lenient about visas for others from Egypt, Lebanon, and Saudi Arabia. Al Qaeda knew the holes in our system and exploited them.

Also, when U.S. flight schools reported oddities about Arabic student pilots, the FAA showed little concern. When the FAA was told that future hijacker pilot Hani Hanjour was so poor in English that it was dangerous to let him fly aircraft, the FAA suggested providing Hanjour with an English language tutor.

Equally puzzling was the FAA's seeming disinterest when two of the future hijackers, barely out of flight school, left their plane in the busy traffic at Miami airport. The FAA bawled out the plane's owner but did not investigate the pilots.

"The fact that that incident didn't cause some bells to ring is hard to believe," Sen. Graham asserts in hindsight. "If those bells had rung, it might have led to a series of investigations that would have peeled back the plot that was beginning to develop and potentially could have avoided Sept. 11."

That same failure to perceive real danger afflicted ordinary Americans, too: some flight schools not bothering to demand the right kind of visas from foreign flight students; a martial arts trainer instructing a hijacker in hand-to-hand combat, including the use of knives, without questioning the motive; a motel manager suspicious of the tense urgency with which two hijacker pilots demanded untraceable, 24-hour Internet access telling his boss but not the FBI.

In short, as the Sept. 11 plot built to its horrendous climax, it was our mindset that was America's Achilles Heel -- our disbelief that the U.S. could be reached by Al Qaeda terrorists smart, sophisticated, and dedicated enough to strike us at home. This mindset made it hard to read the warning signs.

We can only hope now that part of what lies buried beneath the ashes at Ground Zero are America's illusions that we can be safe while continuing to ignore large parts of the world and without working to understand what kind of people are so alienated from America -- and why -- that they would die to destroy us.