The Squad 5 agents dropped everything to pursue the Moussaoui case. When they discovered his visa had expired, they locked him up. They scoured his background, questioned his friends and roommate, and sent out requests to foreign governments for any records they might have on him. French intelligence came back with a tantalizing lead: Moussaoui was known to have “radical Islamic” beliefs and ties to followers of Osama bin Laden in Chechnya.

Certain they were on to something big, in early September the agents asked Washington for a special national-security search warrant, which would allow them to open a computer disk seized when they detained Moussaoui. But as NEWSWEEK first reported, the FBI’s lawyers turned them down. There wasn’t enough information to show Moussaoui was an “agent” of a foreign government or terrorist group. The investigation stalled. A few days later came September 11. The Squad 5 agents were devastated, believing they might have uncovered the plot if they’d had more time–and backup from Washington.

Last week the Justice Department showed just how close the agents might have been. In a sweeping indictment, the government alleges that Moussaoui was indeed part of the terrorist plot; the Feds believe he was supposed to have been the 20th hijacker. Claiming Moussaoui was an “active participant” in bin Laden’s conspiracy to murder Americans, Attorney General John Ashcroft called the indictment “a chronicle of evil.” (Moussaoui says he is innocent. “He’s definitely going to fight this case,” said his lawyer.) But in private, Justice officials are frustrated that, despite months of intensive investigation, the only person in the United States they’ve been able to connect in any way to the attack is a man they had in custody before it happened.

Despite the debate over secret military tribunals–or, perhaps, because of it–the administration chose to try Moussaoui in federal court, muffling complaints that tribunals would trample the Constitution in the name of national security. Instead, the Moussaoui trial will become a test case for future prosecutions of bin Laden’s network.

The government’s case against Moussaoui is largely circumstantial, but makes for a compelling tale. Moussaoui, now 33, fell under the influence of radical Islam in the mid-’90s while attending business school in England. As early as 1994, a French magistrate warned British intelligence about a young radical named “Zacarias.” The Brits never followed up. The French tracked Moussaoui to Afghanistan, where they believe he trained in a Qaeda camp.

This information might have been useful to U.S. intelligence in the fall of 2000 when, according to the indictment, Moussaoui began trying to enter the United States on a student visa. Most investigators think Moussaoui was Al Qaeda’s second choice to round out the hijacking team slated for United Airlines Flight 93–signed up only after suspected terrorist Ramzi bin al-Shibh failed to get into the United States. A top Qaeda operative based in Hamburg, Germany, al-Shibh tried four times to enter the country on a student visa but was rejected each time, perhaps because he hadn’t been accepted by any U.S. schools.

Avoiding al-Shibh’s mistake, Moussaoui enrolled in the Airman Flight School in Norman, Okla., and was approved for a student visa. When he arrived on Feb. 26 of this year, admissions director Brenda Keene recalls, Moussaoui was a “pain in the rear.” He asked so many irritating questions on the first day, Keene told NEWSWEEK, “I put my hands around his throat and told him, ‘You’re driving me crazy!’ "

Moussaoui was a truly miserable flier who kept a death grip on the controls. Oddly, Moussaoui poured out his frustrations to his landlord. “He told me he didn’t think he was a natural,” Sol Ward told NEWSWEEK. Eventually, his teachers gave up and grounded him. He turned up three months later at the Minnesota flight school, where his suspicious teachers turned him in to the Feds.

The indictment details how Moussaoui’s behavior closely paralleled some of the other hijackers’. He joined a gym, bought knives and purchased flight videos from the same store as the others. When the Squad 5 agents finally got their search warrant after September 11, they found materials on Moussaoui’s computer disk that explained how to set up a crop-dusting company and technical data about how the wind spreads pesticides. Lead hijacker Mohamed Atta had also made inquiries about crop-dusting. The Feds found evidence linking Moussaoui to al-Shibh, Atta’s former roommate in Hamburg. The indictment charges that less than two weeks before Moussaoui arrived in Minnesota, al-Shibh had wired him $14,000. Agents searching Moussaoui’s apartment found al-Shibh’s phone number.

Still, the indictment does not allege any contacts between Moussaoui and any of the 19 other hijackers. “There’s no direct testimony in this case,” says a Justice official. “It’s all circumstantial.” The other hijackers met and traveled together. Moussaoui seems to have acted solo. Why? “We don’t know the answer to that,” says a senior law-enforcement official. Defense lawyers could also try to argue that Moussaoui didn’t know the details of the attack–since bin Laden said, on the videotape released last week, that many of the hijackers were unaware of their mission until just before boarding the planes.

NEWSWEEK has learned that the Feds have intelligence linking Moussaoui to at least one of the other hijackers, Hamza Alghamdi, whose plane crashed into the second tower. Foreign intelligence sources told the U.S. government that Moussaoui and Alghamdi were “close associates” and may have met in a bin Laden “guesthouse” in Afghanistan. But prosecutors left that point out of the indictment, for reasons that are unclear.

Since September 11 the Justice Department has loosened the tough rules governing surveillance and wiretaps. NEWSWEEK has learned the Feds are tracking at least 76 individuals nationwide who have suspected ties to bin Laden. Nearly 20 of them are being closely monitored under special national-security warrants. The expanded eavesdropping may not rid the country of bin Laden’s acolytes, but it could keep the next Moussaoui from slipping away.