While the vast majority of British Muslims abhor last week's terror attacks in London, there are a vocal few who have found Britain to be a safe haven for holy war.
Abu Hamza al-Mazri -- a one-eyed firebrand who has a hook to replace an arm he lost fighting in Afghanistan -- preached hate for years at Finsbury Park mosque in northern London. His trial on terror-related charges was supposed to start last week.
Al-Mazri's known disciples included Zacarias Moussaoui, the so-called 20th 9/11 hijacker, and Richard Reed, who allegedly tried to blow up a trans-Atlantic jetliner with explosives hidden in his shoes. Both were London residents.
"Not for nothing, it is known as 'Londonistan' by those who watch the movement of radicals," said Steven Simon, a senior analyst at RAND Corporation, a nonprofit research organization.
Some, like al-Mazri, fled repressive regimes in their home countries and found asylum in Britain.
Others are natural born citizens. Last year, British security officials found stockpiles of ammonium nitrate and other ingredients used to make a fertilizer bomb near London's Heathrow Airport. The suspects, said police, were British citizens who were Musilm.
Prior to the 9/11 attacks, it was legal in Britain for militants to promote armed struggle as long as it was aimed at other parts of the world.
The British government knew there were at least 100 hardcore supporters of Osama bin Laden, but the government chose, in many cases, to watch, rather than arrest them.
Many of the new generation of British-born militants are believed to be self-selected and self-taught by downloading terrorist training manuals off the Internet.
"People even in junior high school -- 14 to 15 years old -- have been identified as conspirators," said Robert Leiken, director of the Immigration and National Security Programs at The Nixon Center in Washington.
Even today, investigators said they assume other terror cells are still operating in Britain, aside from the group that perpetrated last week's attacks in London. But officials vowed to remain vigilant.
ABC News' David Wright filed this report for "World News Tonight."