By Frank Gardner
Security correspondent, BBC News
Robert Mueller outlined what he sees as a three-tiered threat from al-Qaeda
The head of the FBI has said he believes the West can achieve victory over al-Qaeda within three-and-a-half years.
Robert Mueller described how his organisation is working closely with British intelligence to confront ever-more-complex plots.
Flanked by broad-shouldered security men with tell-tale bulges beneath their suits, the director of the FBI gave a rare public address in London.
As head of one of 16 US intelligence agencies, Mr Mueller is at the forefront of preventing a repeat of the September 11 attacks.
It was a task, he said, which could not be done without strategic partnerships with allies like Britain.
Britain's role 'key'
He said the West was now confronting a three-layered threat from al-Qaeda, but he believed it could be defeated, as he put it, on his "watch".
The top tier was the core of the organisation which had established new sanctuaries in Pakistan's tribal areas.
The middle tier was the most complex, consisting of small, self-directed groups like the London bombers of 7/7 who had some ties to al-Qaeda's leadership.
The bottom tier, said Mr Mueller, was made up of homegrown extremists who met on the internet instead of in foreign training camps.
He said the key to confronting these threats lay in good human intelligence sources, wire-taps or eavesdropping, and international partnerships.
The vast majority of the FBI's terrorism cases, he said, originated from information developed by US partners overseas, particularly Britain.
Mr Mueller said FBI personnel had more than 20 face-to-face meetings every week with British counterterrorism officials in London.
He said: "We never know when a fragment of information uncovered in one country could unearth an entire network of terror in another."