More than a year before 9/11, a Pakistani-British man told the FBI an incredible tale: that he had been trained by bin Laden’s followers to hijack airplanes and was now in America to carry out an attack. The FBI questioned him for weeks, but then let him go home, and never followed up. Now, the former al-Qaida insider is talking.

In March, 2000, Niaz Khan said he was down and out, waiting tables in a curry house north of London, overwhelmed by gambling debts and increasingly drawn to the message of a radical local imam. The imam extolled Osama bin Laden and the rewards of dying for jihad.

Then, one night, outside a casino in Manchester, England, Khan said two mysterious men approached him. “First they say, ‘We can help you,’" recalls Khan. "I say, ‘How can you help me?’ Say, ‘OK, come sit in car.’ Said ‘Do you heard Osama name’?”

Khan, now 30, said the men told him they were working on behalf of Osama bin Laden, knew all about his background and gambling debts — information presumably gleaned from his fellow mosque members — and offered to teach him the ways of jihad.

They gave Khan several thousand dollars and flew him to Lahore, Pakistan, where he waited for instructions in a local hotel. He says that bin Laden’s followers then drove him, blindfolded, to a nearby safe house.

In training

Khan told NBC News that for the next few weeks he was trained by al-Qaida to hijack passenger planes, and then sent to the United States. But when he told the FBI, headquarters was skeptical and, after several weeks, senior FBI officials ordered him released to the custody of British intelligence. Khan said, “I told them before the 9/11, about more than year, be… hijacking in America or on America airline.”

Khan said that at the Lahore training compound he and up to 30 other men were taught hijacking basics, including how to smuggle guns and other weapons through airport security, techniques to overpower passengers and crew and how to get into a cockpit.

Khan says he did not think about all the other people he might have killed and, at the time, didn’t care. “Not that time," he said. "If I die, it doesn’t matter because this life anyway, it’s no good.”

After about a week of training, Khan said he was given money to fly a circuitous route from Pakistan to Doha, Qatar, to London, to Zurich, Switzerland, back to London, and then off to New York. The purpose, he said, was to allow him to observe flight operations and on-board security measures.

Upon landing at JFK airport, Khan says he was supposed to go to a taxi stand, find a man in a white prayer cap and use a code. “He say, ‘Your name Babu Khan?’ " said Khan. "And you will be saying, ‘Yes, my name Babu Khan.’ ‘Your name Babu Khan?’ You say, ‘Yes, my name Babu Khan.’”

But Khan claims he got cold feet. Instead of meeting his contact, he slipped away, retreated to New York, then took a bus to Atlantic City and gambled away almost all his money. Fearful that he had blown al-Qaida’s cash, and aware that his terrorist trainers had copied his passport information and easily knew how to find him, Khan turned himself in and confessed. “I’ve been to Pakistan," he said. "I know about this hijacking, something going on.”

Khan said his trainers never told him exactly what his terrorist mission in the United States would be. He said he was told he would learn more details from a half dozen other trained terrorists who, he was told, already were in the U.S.

For three weeks, FBI counter-terrorist agents in Newark, N.J. interrogated Khan, created composite drawings of his terrorist trainer and a fellow student and then wired Khan up and took him back to JFK airport, hoping to smoke out other conspirators. But they had no luck.

Lie-detector tests

Congress’ 9/11 report confirms that in April, 2000, an unnamed “walk-in” told the FBI he “was to meet five or six persons” — some of them pilots — who would take over a plane and fly to Afghanistan, or blow the plane up. The report adds that the “walk-in” passed a lie-detector test.

NBC News has learned that Khan passed not one but two FBI polygraphs. A former FBI official says Newark agents believed Khan and tried to aggressively follow every lead in the case, but word came from headquarters saying, “return him to London and forget about it” -- which, critics say, is exactly what the FBI did.

But the FBI insists it investigated Khan’s allegations thoroughly, could not confirm them, and had no legal grounds to hold him. Federal prosecutors agreed. FBI officials say they did the right thing in turning Khan over to British authorities, and assumed they would carefully investigate.

But NBC News has learned that New Scotland Yard only interviewed Khan for about two hours, and then released him. Spokesmen for Scotland Yard and MI-5, the British intelligence agency, would not comment.

Khan said he watched the 9/11 attacks on television and was horrified. He said he was sad for the victims and relieved he had not carried out any attacks of his own. To him, the 9/11 plot rang familiar. “Maybe same plan," he said. "Maybe same training.”

There’s no evidence Khan was part of the 9/11 plot. But lawyers for 9/11 families urged him to tell his story, arguing it reveals a major missed clue.

Khan says the British tabloids offered to pay him to tell his story, but he declined. He wants it known that he has not accepted any money for any interview.

He is fearful for his life and, at first, was reluctant to talk to NBC. He changed his mind after a British newspaper published his name and, he says, surreptitiously took his photograph. Once it was published, he agreed to go on camera to talk about what he sees as a missed opportunity.

Khan remains surprised that, to this day, the FBI, CIA and Scotland Yard have never asked for his help in identifying the street address of the Lahore safe house where he and dozens of other men were trained. He says he saw some identifying signs and might be able to locate it today. “I just surprised because [they] never come back to ask some more things," he said. "[The FBI] believed me, but maybe not seriously.”

Now that he’s told his story, Khan plans to go back into the shadows—branded by some a terrorist and by others a traitor to the cause.

Lisa Myers is NBC News’ Senior Investigative Correspondent