Read related reports from The New York Times and the Canadian Broadcasting Corporation.

In the aftermath of the 9/11 attacks, the locus of the investigation quickly shifted to Europe and the network of radical Islamic jihadis who are part of "Eurabia," the continent's expanding Muslim communities. Since 9/11 America has been spared what authorities feared and expected: a second wave of attacks. Instead Europe, once a logistical base for Islamic radicals and a safe haven, has itself become the target.

On March 11, 2004, the Islamic jihadis made it clear once and for all that there was a new front in their war. Madrid's morning commuter trains were ravaged by simultaneous explosions. Nearly 200 people were killed and thousands injured. Only a faulty detonating device saved thousands more from death in the packed central train station. Since 9/11, European law enforcement and intelligence agencies have foiled dozens of Islamist terrorist plots. In "Al Qaeda's New Front," FRONTLINE, The New York Times and the Canadian Broadcasting Corporation's documentary program the fifth estate join forces to investigate the realities of "Eurabia," and the peculiar problems faced by Western governments in confronting this gathering threat.

The key reality faced on the other side of the Atlantic is the 18 million Muslims whose ranks are expected to swell to 20 percent of Europe's population in the next 15 years. This community of immigrants who share religious and ethnic bonds has largely failed to integrate into European societies. Many are poor and subject to bigotry; they have lived in Europe for years and many were born there, yet often feel that they are not full members of society. This sense of alienation is deepened by the ubiquity of television with its non-stop images of their suffering brethren in Palestine, Iraq, and Chechnya. Inspired by local radical imams and jihadist Web sites, disenfranchised European Muslims are taking up the cause of jihad.

With full-scale war between the U.S. military and Islamic insurgents in Iraq -- which is just a two-and-a-half day drive from Berlin -- the reality of a war between Islam and the West is a domestic problem for Europe. The dream of the European Union, the end of all borders, has had unintended consequences. It means that a terrorist can travel freely once he has gained entry, leaving law enforcement with the nearly impossible task of tracking clandestine warriors as they slip in and out of countries with literally no restrictions.

That ease of movement presents America with an ongoing threat: a visa waiver program that makes travel by any citizen or permanent resident of Europe into the United States virtually unrestricted.

Since 9/11, intelligence sharing between the United States and most of Europe's governments has reached unimagined levels. But within the European Union itself difficulties persist as each country continues to have its unique laws and civil rights protections.

While Europe girds itself for more attacks, all of the top counter-terrorism officials interviewed for this report warn that the threat is only growing -- in part, they lament, because America's strategy of going to war in Iraq has created a new intense threat from combat-hardened veterans of that insurgency and a large immigrant population with growing sympathy for their cause.

From Bali in the Pacific to Beslan in remote Russia, the images deliver a stark message: nobody is safe in a war without borders -- a war now threatening to boil over in the heart of Europe.