A flight instructor who raised the alarm about the so-called “20th hijacker” has been given a $5 million (£2.5 million) reward by the US government even though his tip failed to prevent the September 11 terror attacks.

Clarence “Clancy” Prevost, a former US Navy pilot who taught at the Pan Am International Flight Academy outside Minneapolis, became suspicious of Zacarias Moussaoui when he wanted to learn to fly a jumbo jet without showing any interest in take-off or landing.

The French national - the only person ever convicted in the United States for the September 11 attacks - was arrested on immigration charges but Minneapolis FBI agents were unable to persuade their superiors to seek a national security warrant to search his belongings and laptop computer.

He sat in jail for 3 and a half weeks saying nothing until 19 hijackers seized control of four airliners in the coordinated 2001 attack on the World Trade Centre and the Pentagon.

Moussaoui later confessed to being the “20th hijacker” and told jurors he was to have piloted a fifth plane into the White House. After he was sentenced to life in prison, however, he recanted his testimony and denied any role in the attacks.

Mr Prevost, 69, a retired Northwest Airlines pilot, testified that he learned by the second day of teaching Moussaoui that he had paid the bulk of his $8,300 tuition for the flight simulator course in cash with $100 bills.

Although he described Moussaoui as a “pretty genial guy”, his concerns were heightened when the would-be pilot raised his voice when asked about Moslems’ Hajj pilgrimage. “Are you Muslim?” Mr Prevost asked. “I am nothing!” Moussaoui angrily replied.

Moussaoui’s stated goal was to learn to fly from London’s Heathrow airport to John F. Kennedy airport in New York. But he had no pilot’s licence and only about 50 hours flight time on a single-engine propeller plane - a fraction of the 600 hours of most students.

Mr Prevost testified that Moussaoui “had no frame of reference whatsoever with a commercial airliner. After 15 minutes I said, ’Let’s get lunch.’“

He told his managers: “We don’t know anything about this guy, and we’re teaching him how to throw the switches on a 747.” The managers initially responded that Moussaoui had paid his money and they did not care. Mr Prevost responded: “We’ll care when there’s a hijacking and the lawsuits come in.” Mr Prevost received a pay-out at a private ceremony yesterday under the US government’s “Rewards for Justice” programme after the award was secretly authorised last autumn - even though Moussaoui was never named as a wanted suspect by the programme.

The reward shocked two other Pan Am flight instructors - Tim Nelson and Hugh Sims - who have also been lauded for tipping off the FBI about Moussaoui.

“He was certainly there but he didn’t call the FBI. I have no idea why he received the reward,” Mr Sims said.

Mr Nelson’s wife Jodie said the reward “was given out to the wrong person” and described her husband as “dumbfounded”.

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