(CBS) The man who has just written the most comprehensive study ever done on al Qaeda says the terror group has been planning a "dirty" bomb attack for years and has made several serious and expensive attempts to acquire nuclear material.

In an exclusive interview with CBS News Correspondent Mark Phillips, Rohan Gunaratna, Author of "Inside al Qaeda," said that $1.5 million was paid by al Qaeda to a Sudanese military officer who provided a uranium canister that originated from South Africa.

But the canister turned out to be empty. So while it continued it's training, al Qaeda continued to seek nuclear material through Russian and Ukrainian underworld figures and from Pakistani nuclear scientists. And it's still trying.

"An organization that has tried for such a long time will be successful. If not in the immediate, then in the mid-term and that is the real reason why this organization must be disrupted," said Gunaratna.

His book will be published later this week. Among its other revelations is the fact that the September eleventh attacks were supposed to be even bigger – al Qaeda was also targeting the British Houses of Parliament, for what would be an international display of terrorism's reach.

According to Gunaratna, "This team assembled at the Heathrow airport on 9/11 to conduct an airborne suicide attack on the houses of parliament."

But the al Qaeda operatives, he said, hadn't planned on one contingency: that after the U.S. attacks, all flights would be grounded. And so there were no flights for the team to hijack.

The testimony of a man named Afroz Mohammed is cited as proof of the planned attack. He was arrested in India after fleeing Britain, and has admitted to the London plan in an Indian Security services document obtained by CBS News.

Afroz has since retracted his confession, but his training followed a familiar pattern. In addition to attending flying schools in Britain, Australia and the U.S., Gunaratna said Afroz received money from al Qaeda-connected sources.

"In terms of who recruited him, how he was recruited where he was sent for training, it very clearly demonstrates that he was an al Qaeda member and this was his intention," Gunaratna explained.

For his book, Gunaratna studied intelligence documents and had rare access to serving and former members of Osama bin Laden's organization. He found a "greater willingness on the port of both terrorists, supporters, sympathizers to talk to us."

What he said he learned – and what the American intelligence failed to understand – is al Qaeda's "lose and learn" doctrine. Al Qaeda was prepared to lose terrorists like Ramzi Ahmed Yousef, the 1993 World Trade Center bomber, in order to learn how to carry out the attack successfully the next time.

"The Americans should have known that the terrorist organization that was behind Ramzi Ahmed Yousef, that trained him, that supported him -- this organization would never abandon that idea," he said.

And Gunaratna noted, al Qaeda is still operating, and still planning.