As details emerge about the terrorists who struck London's mass transit system last week, many people are questioning how four young men who had lived in Great Britain all or most of their lives could turn into fanatics capable of killing their fellow Britons for the cause of radical Islam. Although their names have not yet been released, three of the men are believed to have lived in a predominantly Muslim neighborhood in Leeds, some 200 miles north of London. Yet these men considered their fellow countrymen infidels and enemies whom they wanted to kill in the most vicious manner. Could the same thing happen here?

 The U.S. has already suffered a devastating attack on September 11, 2001, and may again become a target. A new CNN/USA Today/Gallup Poll shows that 55 percent of Americans believe we will be hit again, perhaps in the next few weeks. But when most Americans think about the terrorist threat, they envision it coming, as it did before, from abroad. None of the September 11th jihadists was American-born or raised, nor were their predecessors, the men who killed six persons and injured many more in the first World Trade Center bombings in 1993. Americans were attacked by men who came here with the express purpose of killing us, not by home-grown terrorists.

 But the London bombings appear to be different. England has a huge problem with large, unassimilated and hostile Moslem communities. According to Peter Bergen, author of "Holy War, Inc.: Inside the Secret World of Osama bin Laden," a British government report last year estimated that 10,000 to 15,000 British Muslims support al Qaeda and other terrorist groups, and somewhere between 300 and 600 of these trained in terrorist camps in Afghanistan before the Taliban was overthrown. Even in the face of this threat, Britain has been reluctant to crack down on the most extreme hate-mongers in its Muslim populations. Only after tremendous pressure, for example, did the government decide to prosecute a radical cleric, Abu Hamza al-Masri, for inciting murder, though he had openly preached violence for years to young men -- including Zacarias Moussaoui, the so-called 20th hijacker, and attempted shoe-bomber Richard Reid -- before police finally arrested al-Masri last year.

 As Middle-East expert Daniel Pipes reported recently, "British-based terrorists have carried out operations in Pakistan, Afghanistan, Kenya, Tanzania, Saudi Arabia, Iraq, Israel, Morocco, Russia, Spain, and America. Many governments -- Jordanian, Egyptian, Moroccan, Spanish, French, and American  -- have protested London's refusal to shut down its Islamist terrorist infrastructure or extradite wanted operatives." So why has Great Britain gone wobbly in cracking down on radical Islamists in its midst? Pipes gives some insight in describing an incident at Denbigh High School in Luton, a town 30 miles outside London -- a site where Tuesday police found a car loaded with explosives that may be related to the London bombings.

  "Denbigh," writes Pipes, "has a student population that is about 80 percent Muslim. Years ago, it accommodated the sartorial needs of their faith and heritage, including a female student uniform made up of the Pakistani shalwar kameez trousers, a jerkin top, and hijab head covering. But when a teenager of Bangladeshi origins, Shabina Begum, insisted in 2004 on wearing a jilbab, which covers the entire body except for the face and hands, Denbigh administrators said no."

 The case ended up in court, and Ms. Begum won the right to wear the jilbab, hailed at the time as "a victory for all Muslims who wish to preserve their identity and values despite prejudice and bigotry" by her lawyer at the appellate level -- none other than Prime Minister Tony Blair's wife, Cherie Booth.

 Britons seem to have given up on assimilating their Muslim population, with many British elites patting themselves on the back for their tolerance and multiculturalism. Meanwhile, an increasing number of Muslim youth in Great Britain grow more alienated and hostile to their adopted country. Obviously there is a big leap from wearing the jilbab to blowing up trains, but it would be naive not to recognize that a fanatical brand of Islam -- unchallenged and unchecked -- is at the root of both.