You find them all over Europe, on the outskirts of big cities everywhere: neglected suburbs ranging from the modest to the poor, where immigrant families have made their homes.
In Spain those families are mostly Moroccan; in France they have come from all over North Africa; in Germany they are mainly of Turkish origin; in England of Pakistani, Indian, or Bangladeshi background.
But they all have one thing in common, police investigators are discovering. They offered an ideal environment for Al Qaeda operatives seeking to blend into their surroundings and thus evade detection by the authorities.
The latest round of terrorist suspect arrests in Europe - last week's sweep caught 16 men and one woman in Leicester, central England - was a case in point. Leicester's 35,000 Muslim citizens have built themselves 19 mosques and Islamic centers. In some of them, Al Qaeda agents apparently spread their message and found new recruits.
"Parts of Europe have significant visible minorities, people that Al Qaeda could merge naturally with," says Michael Levi, professor of criminology at Cardiff University in Wales.
"Like organized crime groups, they could blend into routine activities."
Since Sept. 11, European investigators have uncovered a picture of their continent that they had barely glimpsed before - as a key refuge, logistical base, and organizational headquarters for Al Qaeda.
Police have arrested scores of suspected Al Qaeda members in France, Italy, Germany (where the ringleaders of the Sept. 11 hijackers lived), Belgium, Holland, Spain, Bosnia, and Britain.
Europe offers more than just ethnic camouflage, however, to North African or Middle Eastern terrorists.
European integration has helped them. In the European Union, which has done away with most border controls, they can slip from one country to another without risking passport checks, for example.
And the lack of full integration has helped them too, since police and judicial agencies remain under national control. "They have traded on coordination weaknesses in intelligence in different countries," says Prof. Levi. "People never put the whole picture together."
Geographically, Europe sits in a strategic part of the world, with good communications links to the Middle East, Asia, and the United States. It offers attractive targets itself for terrorists: French police have uncovered what they say was a plot led by alleged Al Qaeda member Djamel Beghal to blow up the US Embassy in Paris, and Italian police last year arrested several men in connection with a plan to blow up the European Parliament in Strasbourg, France.
At the same time, says Magnus Ranstorp, an expert at the Center for the Study of Terrorism and Political Violence at St. Andrews University in Scotland, Al Qaeda cells can take advantage of "the lack of intrusiveness of the authorities into ordinary peoples' lives."
Radical Islamist clerics have been able to preach "jihad" openly in mosques in many European countries, protected by free speech laws.
That is especially true of Britain, where the alleged "20th hijacker" from the Sept. 11 attacks, Zacarias Moussaoui - a French citizen of Moroccan origin currently facing trial in the United States - is reported to have been recruited. Lotfi Raissi, an Algerian whom US authorities suspect trained some of the Sept. 11 hijackers, lived quietly outside London for several years. Richard Reid, the "shoe bomber" arrested in December as he allegedly tried to blow up a plane flying from Paris to Miami, seems to have been converted to radical Islam while in jail in London.
Earlier this week, a French newspaper reported that Reid was in e-mail contact with supporters across Europe before he boarded the US-bound flight. Reid reportedly sent a message - later found on a hard drive in a Paris cybercafe - in which he claimed to be a "martyr for the Islamic cause."
Police and intelligence agencies in Europe have not been unaware of the potential danger in their midst. The Spanish authorities say they have been monitoring extremist Islamic groups for a decade. But rivalries and different priorities between different agencies have complicated their task.
"It is a balancing act between not disturbing cells because they could lead to further arrests," which tends to be an intelligence agency's approach, and making arrests as soon as sufficient evidence has been collected, which is traditionally the police attitude, says Dr. Ranstorp.
That balance has now changed, adds Levi. Since Sept. 11, the European authorities "have adopted a preventative line, to step in. 9/11 changed the rules of the game. A clear and present danger has become part of the mental set."
Still, warns Ranstorp, the police are up against networks skilled in counterintelligence and concealing themselves - using e-mails sent from public Internet cafes rather than telephones to communicate, for example - which means that "the real problem is getting and using real-time intelligence."
Two allegedly important members of Al Qaeda, its ambassador to Europe Abu Qatada and the man suspected by the FBI of financing the terrorist cell that flew a plane into the Pentagon, Omar al Bayoumi, have so far escaped British police dragnets.
Even if they are caught, however, putting them behind bars may prove to be especially difficult, given the delicate nature of the evidence against them. Intelligence agencies, both European and American, are often reluctant to reveal anything that might shed light on how they gather their information.
Six Bosnian terrorist suspects were released from jail in Sarajevo last week because the US authorities would not present evidence against them that could be used in court (though they were handed over to US soldiers and are now en route to the US naval base at Guantánamo Bay, Cuba). The British police are said to fear that they will not be able to hold and prosecute Mr. Raissi because of Washington's reluctance to provide usable evidence against him.
In the United States, the authorities plan to sidestep this difficulty with military tribunals, which will have lower evidentiary standards than traditional courts.
But in Europe, where no such tribunals have been created, the lack of public evidence is "a huge problem," says Ranstorp.