CNN, Friday, March 3, 2006 Posted: 0419 GMT (1219 HKT)

MINNEAPOLIS, Minnesota (CNN) — He spoke fluent Arabic but rusty English. He had plenty of cash, but didn’t seem like the playboy type. He said he wanted to learn to fly a jumbo jet simply to impress his pals.

But when al Qaeda operative Zacarias Moussaoui asked a flight instructor how to turn off the oxygen and transponder on a jet, two managers at the flight school had a hunch something was up.

That hunch may be the reason that Moussaoui — the only person indicted in the U.S. in connection with the September 11, 2001, terrorist attacks — is awaiting a death penalty trial next week.

The managers — Hugh Sims, 65, and Tim Nelson, 45 — said they saw red flags before Moussaoui even showed up at the Pan Am International Flight Academy in Eagan, Minnesota, 29 days before the attacks that toppled the World Trade Center and left a smoldering hole in the Pentagon. (Watch the men who suspected Moussaoui — 6:53)

Those instincts earned the men, both Air Force veterans, a Senate proclamation last year commending them for their bravery that "possibly prevented another attack against our nation."

It was an e-mail from Moussaoui to the flight school’s Miami, Florida, headquarters that first piqued their suspicion. In it, Moussaoui — using the handle "zuluman tango tango" — said he wanted to learn how to fly 747 passenger jets.

"I need to know if you can help me achieve my ‘goal,’ my dream," Moussaoui wrote, listing five types of Boeing and Airbus jets. "To be able to pilot one of these Big Birds, even if I am not a real professional pilot."

Moussaoui further claimed to be a British businessman, and in the e-mail — laced with grammatical errors — he said he wanted to learn how to take off and land, communicate with air traffic controllers and navigate between London, England, and New York City.

But Moussaoui had no pilot’s license and only 55 hours of flying time on small aircraft at a flight school in Oklahoma. He had never flown solo.

"I know it could be better, but I am sure you can do something. After all, we are in AMERICA, and everything is possible," Moussaoui wrote.

Nonetheless, Moussaoui was allowed to sign up for classes and flight-simulator training for the Boeing 747. He paid $1,500 of the $8,300 for the class with a credit card, the rest with cash when he showed up at the school.

Nelson said he was suspicious because cash is so difficult to track, but he set aside his concerns, figuring the man was a wealthy thrill seeker.

However, when Moussaoui, who claimed to be an international consultant, arrived wearing a T-shirt, jeans and a baseball cap, Sims was confused.

"I was expecting this guy to show up very well-dressed," Sims said. "He just didn’t fit the profile of what I would think he would be, for want of a better word, a rich playboy type."

Sims said Moussaoui’s English skills were fair, but didn’t seem up to snuff for an international businessman.

"His demeanor was not that sophisticated," Sims said.

He also had a weird reason for wanting to learn to fly a jumbo jet, said Nelson — he told them that he merely wanted to be able to boast to his friends that he could fly a 747.

"He was telling us that it’s an ego thing," Nelson said. "That’s a lot of money to spend to play."

But Moussaoui’s overall demeanor — often characterized as brash and abrasive because of his condescending remarks to the judge and attorneys involved in his case over the past four years — was friendly, even shy, Sims and Nelson recall. (Read how Moussaoui was barred from the courtroom for his antics)

The next day — August 14, 2001 — Moussaoui continued to earn the suspicion of the flight school staff, first when Nelson saw some Syrian airline students speaking to Moussaoui in Arabic. The students later told Nelson that Moussaoui, who was born to Moroccan parents, was a native speaker.

"That bothered me," Nelson said. "It’s just one more red flag."

At a manager’s meeting at the school, instructor Clancy Prevost said that Moussaoui had asked him how to turn off the oxygen in the passenger cabin and how to disconnect the transponder used to track the plane.

"There was an unease that was beginning to spread out among all the people who had come into contact with him," Sims said.

Nelson had recently seen a training video about a 1999 hijacking in Japan in which a deranged hijacker stabbed a pilot to death and took over the controls of a 747 with 500 people aboard to prove that he could fly.

"I’m thinking, do I have that or do I have something worse on my hands?" Nelson recalled, but he and Sims were both reluctant to call authorities when they had yet to pinpoint anything illegal Moussaoui had done.

But the next day, August 15, they both called the FBI without knowing the other had the same inclination.

"I don’t know what this guy is up to, but he is paying a lot of money for nothing he can use legitimately," Nelson recalls telling the FBI. "You need to understand this aircraft weighs 900,000 pounds. It carries between 50 and 57,000 gallons of jet fuel.

"If you fly it at 350 knots into a heavily populated area, you’re going to kill a boatload of people."

Sims called later and spoke to the same agent, telling him Moussaoui wanted training "that could become dangerous."

The following day, FBI and immigration agents went to Moussaoui’s hotel and arrested him. He had no visa, and his French passport allowed him to stay in the country only 90 days. He had exceeded it.

Moussaoui was indicted in December 2001 in connection with the 9/11 attacks, and upon pleading guilty to terrorism conspiracies last year, Moussaoui revealed he had been less than forthcoming when he told flight instructors his dream was to "pilot one of these Big Birds."

In fact, Moussaoui told the court, his dream was to pilot a big bird right into the White House in what he hoped was a second wave of attacks to follow the ones in New York and Washington.

"Maybe we did stop something from happening," Nelson said. "I was hoping I was wrong, because being right — we saw what being right was — 9/11."