The McClains said they knew Moussaoui as “Zac” and said he was always alone. “We didn’t ever see him with anyone,” Rob told NEWSWEEK. “We just said, ‘Hi.’ It wasn’t ever like a conversation.” They also told the agents something the FBI already knew–that Moussaoui had been gone for months.

In May, they said, Moussaoui piled everything he owned into his “junker” white Ford Taurus and driven away, leaving no forwarding address. The McClains were stunned to learn that Moussaoui, a 33-year-old French citizen who had come to Norman to attend flight school, was suspected of involvement in the terror attacks they had just seen on TV.

Now they are aware that Moussaoui has been linked to the Sept. 11 attacks because of phone calls he allegedly made to the Hamburg, Germany, apartment of chief suspect Mohamed Atta. They also know that after Moussaoui left flight school in Norman, he moved on to Eagan, Minn., to try to learn how to fly commercial jets. It was an instructor at the Pan Am Flight Academy in Eagan who first called the FBI about Moussaoui, reporting that he had a student who wasn’t interested in learning to take off or land big planes but only in how to steer. The FBI arrested Moussaoui on Aug. 17 on immigration charges and is still holding him, although he has now been moved from Minnesota to New York City.

In retrospect, Moussaoui’s strange approach to flying commercial jets suggests that he was part of the Sept. 11 plot, and federal officials have theorized he may have been assigned to the terrorist group that hijacked United Airlines Flight 93, which crashed south of Pittsburgh. The reason: while the other three planes each had five hijackers aboard, United 93 had only four. As a result, investigators have been looking into every aspect of his activities in Norman, where he was a student at the Airman Flight School. Moussaoui first contacted Airman in September 2000, inquiring about a course to qualify for a commercial pilot’s license. Told it would cost $20,000, he asked for information on a private-pilot’s course, which cost $5,000. According to officials at the school, Moussaoui’s inquiry came not long after a similar one from Mohamed Atta. Brenda Keene, the school’s admissions director, says that Atta and Marwan al-Shehhi, another hijacker, toured the school in July 2000. They stayed overnight at the Sooner Hotel on the university campus, where Airman students get a discount, and left without signing up. Keene said there was “nothing out of the ordinary” about their visit.

But Moussaoui showed up for classes on Feb. 26, 2001. He had a beard, a full head of hair and a serious demeanor. He paid half the tuition in cash and asked a lot of questions–so many that the enrollment process, which usually takes about 15 minutes lasted two hours. “He wasn’t really paranoid–more like overbearing,” Keene says. He also needed help with routine business dealings like opening a checking account. Keene asked another school employee to give Moussaoui a hand. That employee got back to the office frazzled. “I’m gonna kill you for making me do that,” she joked. “God, this guy needs to get a life and lighten up.”

Keene saw a faint trace of humor a few weeks after Moussaoui enrolled. “He was joking around and saying, ‘See how I haven’t come to you with any more questions? I am a man of my word!’”

He was also, apparently, a lousy pilot. In early May, when Moussaoui paid the balance of his tuition, he should have been ready to solo in a light plane. He wasn’t, and Dale Davis, the school’s director, had to call Moussaoui into his office to tell him he was grounded. “He wasn’t getting it,” Keene says, “and he was pretty stubborn about it.” Though Moussaoui agreed to hit the books, he never showed up for class after May 29. “He wasn’t where he should have been,” Keene says. “He could have maybe, eventually, got it.”

Instructors say Moussaoui was clearly frustrated by the school’s tough attitude. A woman who met Moussaoui at a local mosque said “a lot of people didn’t like him” because of his brusque attitude, particularly toward women. About this time Moussaoui apparently decided to quit the Airman school and head for Minnesota. He drove there in early August with a friend, Hussein al-Attas, a 23-year-old student whom he’d met at a mosque. Arriving in Eagan, a Minneapolis suburb, Moussaoui and al-Attas took a room together at the Marriott Residence Inn, about a mile from the Pan Am Flight Academy. At Pan Am, Moussaoui–with his intensity and his thick French accent–stood out. He told the school he was interested in learning how to fly the Boeing 747 and pulled out $6,800 in cash as part of the tuition, putting the remainder, $8,500, on an American Express card.

Within weeks, the FBI arrived to take Moussaoui into custody. Now, staff members say, the instructor who called the FBI has left the school fearing retaliation, and the FBI has told the school not to make the instructor’s name public. Moussaoui’s sidekick from Norman, Hussein al-Attas, apparently is also in FBI custody, although the bureau, citing a gag order imposed by a federal judge in New York, will not confirm any details.

Back in Norman, the FBI has interviewed everybody at Airman Flight School seeking details that could help in the investigation. They showed Brenda Keene photos of Mohamed Atta and Marwan al-Shehhi and asked all about Moussaoui’s learning habits, the books he bought and his manner when at the controls of an airplane. “It’s kind of weird,” Keene says. “I mean, this guy who couldn’t even fly a plane supposedly has the mind of a terrorist.”

And 10 days after the terror attacks, Rob and Jennifer McClain received a piece of mail intended for their former upstairs neighbor: a catalog from Sporty’s Pilot Shop, a Batavia, Ohio, mail-order house that caters to pilots. When NEWSWEEK called Sporty’s to see what Moussaoui liked to buy, they said that on June 20, he ordered two “Flight Deck” videos at $34 a piece charged to his Visa card. The videos, each 110 minutes long, put the viewer in the pilot’s seat of the 747-200 and 747-400. They also show the flight engineer’s panel and the navigation system. According to the catalog, the videos provide “an incredibly detailed look at these world-famous airliners”–just what a hijacker would need.