The trial of September 11 conspirator Zacarias Moussaoui provided definitive proof that the U.S. government missed some clear opportunities to stop the 9/11 attacks. Those missed opportunities had nothing to do with the legal restrictions later loosened by the PATRIOT Act. It was bureaucratic hierarchies and power trips that let the Federal Bureau of Investigation ignore the carefully gathered evidence of an attack.

The March 20 testimony of the Minneapolis-based FBI agent Harry Samit buries the notion that 9/11 was unpreventable. Beginning with Moussaoui's arrest on August 16, 2001, Samit mounted a global, indefatigable investigation of the man and concluded that an attack involving hijacked airplanes was imminent.

Michael Rolince, the former head of the FBI's International Terrorism Operations Section, smugly insisted at trial that Samit's "suppositions, hunches and suspicions were one thing," while "what we knew" was another. In short, he said Samit's investigation and leads were not enough: Moussaoui had to speak up for the FBI brass to hear anything.

When defense lawyer Edward MacMahon cross-examined Rolince—possibly the only time a government security official has been so challenged over 9/11—the disconnect between the official story and reality was plain. Rolince testified that he never read the 26-page August 18, 2001 memo Samit had sent to his office warning that Moussaoui wanted to hijack a plane and had the weapons with which to do it. Samit also wrote that Moussaoui "believes it is acceptable to kill civilians" and approves of martyrdom.

A day earlier, Samit had sent an e-mail to his direct superiors recounting Moussaoui's training on 747 simulators. "His excuse is weak," Samit wrote. "He just wants to learn how to do it.…That's pretty ominous and obviously suggests some sort of hijacking plan."

Rebuffed by his superiors and ignored by Rolince, Samit still sought out more information worldwide, from sources as diverse as the FBI's London, Paris, and Oklahoma City offices, the CIA's counterterrorism center, the Secret Service, the Immigration and Naturalization Service, the Federal Aviation Administration (FAA), and probably the National Security Agency. He was sufficiently alarmed by what he heard that he sent an August 21 e-mail requesting the Secret Service be informed that Moussaoui planned to visit the White House and was interested in flight training.

Samit testified that on August 22 he learned from the French that Moussaoui had recruited a fighter to go to Chechnya in 2000 to fight with Islamic radicals with previous links to Osama bin Laden. The FBI brass remained unmoved.

Defense attorney MacMahon then displayed an August 30, 2001, communication addressed to Samit and supervisory FBI agent Mike Maltbie, from a bureau agent in Paris. It said French intelligence thought Moussaoui was "very dangerous" and had soaked up radical views at London's infamous Finnsbury Park mosque. The French also said Moussaoui was "completely devoted" to jihadism and had traveled to Afghanistan.

Yet on August 31 Maltbie stopped Samit from sending a letter to FAA headquarters advising the agency of "a potential threat to security of commercial aircraft." Maltbie said he would handle that, but it is not clear if he ever did. "Minneapolis believes Moussaoui, [Moussaoui's roommate Hussein] Al Attas and others not yet known were…engaged in preparing to seize 747s," the aborted warning said. (Samit did tell Minneapolis FAA officials of his concerns on September 5.)

The information Samit pulled together dovetailed with his belief, based on interviews with the suspect, that Moussaoui had been to Afghan terror training camps. Because he did not have proof of the terror camp connection, Samit never passed this hunch on to FBI headquarters. Maltbie and Maltbie's boss, David Frasca, chief of the radical fundamentalist unit at headquarters, were pressing Samit for facts only.

The 9/11 commission investigation reported that British intelligence directly told U.S. officials on September 13, 2001, that Moussaoui had attended a training camp in Afghanistan. "Had this information been available in late August 2001, the Moussaoui case would almost certainly have received intense, high-level attention," the commission concluded.

As it turns out, Samit strongly suspected that very thing in late August 2001—and could not persuade FBI headquarters to take action. Moussaoui did receive high-level attention: CIA Director George Tenet was briefed on August 23. Nothing came of that.

In 2004, 9/11 commission chairman Thomas Kean told The New York Times he was troubled that word of Moussaoui's arrest never made it to the top of the FBI hierarchy. "If it had maybe there would have been some action taken and things could have been different," he said.

It is now clear that senior FBI officials, Maltbie and Frasca, did know about Moussaoui's arrest. They knew the case so well that they denied Samit's request to seek a warrant from the Foreign Intelligence Surveillance Court to search Moussaoui's computer and belongings. Because Samit never made the explicit link to Afghan terror camps, the FBI could not claim a "foreign power" was directing Moussaoui, the test for an intelligence warrant from the court. But had the bureau taken Samit's fears of mayhem seriously, it could have sought a plain vanilla criminal warrant on Moussaoui based on probable cause. Samit was told that pressing too hard to obtain a warrant would hurt his career.

This decision not to seek a warrant gave rise to the myth that the "wall" between overseas intelligence and criminal investigations made the PATRIOT Act necessary. This myth is cherished among right-wing radio talkers and has now morphed into a clumsy justification for the White House's warrantless wiretaps. It is pure fantasy. Samit cited "obstructionism, criminal negligence and careerism" by top FBI officials—not domestic spying restrictions—as the factors that stopped his investigation.

There is also the bureau's curious flip-flopping on Moussaoui's laptop. In late 2001 FBI Director Robert Mueller told prosecutors there was no information on the computer that linked Moussaoui to the September 11 attacks. At that same time, Rolince himself was not convinced Moussaoui was tied to 9/11. "Whoever that fifth person was is probably still alive," he told reporters at a conference in Europe. "Clearly we are looking into the pool of people who crossed paths with the hijackers." Only later did that someone become Moussaoui.

While Samit spent three weeks trying to get Washington to act on his fears, future hijacker Hani Hanjour was raising suspicions with his flight training in Phoenix. Margaret Chevrette of the Pan Am International Flight Academy reported her worries to the FAA. Somehow those concerns also made their way to CIA chief Tenet and into CIA memos written in August 2001, but the FBI never acted on them. Yet on September 12, FBI agents interviewed Chevrette for more information on Hanjour, reflecting the fact that another local FBI agent (Arizona-based Kenneth Williams) had notified FBI headquarters of the danger posed by terrorists training at U.S. flight schools.

There were also repeated attempts by the New York City FBI office to follow up on hijackers Khalid al-Mihdhar and Nawaf al-Hazmi. Minneapolis, Phoenix, New York—three offices hot on the plot in the days before 9/11. Washington stiffed them all.

Five years later, the bureau still isn't an effective anti-terrorism agency. Instead it's morphing into Stasi Lite, keeping tabs on domestic subversives: assorted peaceniks, Communists in Texas, and the League of Women Voters in Michigan, which had the gall to invite a critic of the PATRIOT Act to a panel discussion. This what the FBI is good at, so this is what it does.

Otherwise, the FBI remains unreformed. The same vapid bureaucratic tautology that made Zacarias Moussaoui unimportant because he was not known to be important still holds sway. Don't be surprised if some other agent is told one day to back off a hunch and get with the program.