"Shy." "Unassuming." "Timid." "Reserved." "Polite." From the descriptions offered by friends, family, and casual acquaintances, one would hardly guess that the three men in question would help plot and execute the most deadly terrorist attack in U.S. history.

Mohamed Atta. Marwan al-Shehhi. Ziad Jarrah. They piloted three of the hijacked planes on Sept. 11. What forces or events transformed these seemingly unremarkable men into fanatical terrorists? And how did their deadly plans for Sept. 11 go undetected for so long?

In "Inside the Terror Network," FRONTLINE correspondent Hedrick Smith explores these and other questions. Interviewing friends, relatives, classmates, and acquaintances, this documentary uncovers the personal histories of the terrorist leaders and traces their movements and plotting in the days, months, and years leading up to Sept. 11.

This report tracks how the hijackers achieved surprise not only by their cunning exploitation of America's open society but also by the failure of intelligence and law enforcement agencies to spot warning signs. Having interviewed top intelligence authorities, Smith reports, "Our side suffered from a failure of imagination. The experts at the CIA, FBI, and INS thought our homeland was safe. They simply did not imagine that foreign terrorists could mount such an attack at long distance and over a long period of time."

Although his name is now infamous, Mohamed Atta, believed to be the plot's ringleader, was just another child growing up in a religiously observant and upwardly mobile household in Egypt. His two sisters were professionals: one a doctor and the other a professor. While pampered by his mother, Atta's father expected him to work hard.

"He studied and he performed his religious duties, just like the rest of the family. We all pray to God and honor our religious duties," his father says.

Atta's family and friends saw nothing unusual or extreme in the young Egyptian. He attended engineering school at home and then, at his father's behest, went to Germany for further study at the age of 24.

"Inside the Terror Network" traces Atta's life in Hamburg -- a magnet for ambitious, young Arabs -- where he later met his future co-conspirators, Marwan al-Shehhi and Ziad Jarrah. It was in Hamburg, authorities believe, that the three students began to become radicalized at the Al Quds mosque.

At his university, Atta organized a prayer group, which included a friendly fellow student with whom he shared an apartment. Marwan al-Shehhi came from the United Arab Emirates, where his father was an imam. Interviewed in the UAE, Royal Highness Sheikh Abdulla bin Zayed said of al-Shehhi, "He was a normal individual until he went to Germany. It seems when he went to Germany, he met these radical fanatics who have a very different understanding of Islam than we do."

This FRONTLINE report chronicles the terrorists' movements from Hamburg to an Al Qaeda terrorist training camp in Afghanistan and eventually to the United States, where they began plotting in earnest the Sept. 11 attack, always operating by Al Qaeda's manual. Along the way the terrorists enlisted the unwitting assistance of numerous Americans: the flight instructor who taught them jet aircraft maneuvers; the fitness trainer who didn't question why a Middle Eastern businessman would need to know hand-to-hand combat techniques for quickly disarming opponents; and the Florida motel manager puzzled by his guests' insistence upon 24-hour Internet access in their room.

"They were very agitated and they said, 'You're wasting our time. We are here on a mission,'" manager Paul Dragomir tells FRONTLINE. "I said, 'What kind of mission is that? A Mission of Islam?' They kind of paused and said, 'No, no, no, we want to stay out of that.'"

"Inside the Terror Network" also tracks the numerous incidents that should have set off alarm bells for authorities: the arrest of an inexperienced flight student in Minnesota, Zacarias Moussaoui, who wanted to learn how to fly a 747 aircraft; wire transfers of large amounts of cash to another terrorist; an incident in which Atta abandoned a small plane on the tarmac of Miami International Airport just days after receiving his pilot license; and Atta's re-entry into the country despite the fact that he had previously overstayed his visa.

"We always had a big problem with the West, convincing them how dangerous these people could be," Sheikh Abdulla bin Zayed says. "Unfortunately, we had to wait until the 11th of September ... for them to understand how dangerous they are."