The towering Ulm cathedral brought pilgrims for centuries to this town on the Bavarian border, but today security officials say a nondescript mosque in an old factory is the new magnet, drawing Islamic extremists around the country.

Intelligence services have been watching the Multicultural House and those associated with it for years, saying they have had contacts with top terror suspects in Germany, including a former top lieutenant of Usama bin Laden.

In coordinated nationwide raids last month, police arrested 15 people on suspicion of belonging to an extremist network allegedly centered on Ulm (search) and neighboring Neu Ulm (search).

Munich prosecutor Martin Hoffmann (search), who led the investigation, said he hopes the arrests will shed light on why these quiet twin cities on the Danube (search) — far from the bustling immigrant neighborhoods of places like Berlin — would become so attractive to extremists.

"There must be someone in Neu Ulm and Ulm that makes it so interesting," he said, refusing to elaborate because the investigation is ongoing.

In the Jan. 12 raids, 700 police officers searched 60 mosques, homes and shops across Germany. Suspects included nationals of Germany, Egypt, Tunisia, Algeria, Libya and Bulgaria; police said their activities included raising money, producing fake passports, and recruiting people for jihad, or holy war. The Ulm area operation focused on the mosque and an Islamic Information Center.

One of the suspects was believed to have been a member in Duesseldorf of Al Tawhid, while others were linked to Ansar al-Islam (search) — both groups with contacts to Jordanian terrorist Abu Musab al-Zarqawi (search), whose supporters are fighting U.S.-led forces in Iraq.

The raids were part of a general crackdown on Islamic extremism in Germany that included the apprehension of three Ansar al-Islam suspects alleged to have plotted to assassinate Iraqi interim Prime Minister Ayad Allawi (search) when he visited Berlin in December, and the arrest of two others at the end of January on suspicion they belong to Al Qaeda.

Ulm's Islamic Information Center, a nondescript storefront within a few hundred yards of the famous cathedral, remained closed a week after the raids. A notice on its window said that lessons at a small school inside had been suspended until further notice.

But the Multicultural House, located between a highway and a scrapyard in an industrial area of Neu Ulm, was bustling as families arrived for a celebration marking the Hajj, or pilgrimage to Mecca in Saudi Arabia. Women wore head scarves, most men were clean-shaven and dressed in modern Western clothing, although some had long beards and traditional Muslim robes.

Though most refused comment, one man in a sport coat and silk paisley vest, who would not give his name, talked about the raids with a sense of incredulity.

"So much has been written that this is a center for extremist Islamists, but it's totally wrong, we're against violence," he said, talking in a stairwell whose door was marked with a sticker saying "Islam is Peace."

Not all members of the center are thought to be radicals. But officials maintain that the Muslim community of Ulm and Neu Ulm, as well as Islamic communities across the country, tacitly support extremists by allowing them to pursue their goals without fear of being reported, and sometimes actively helping them.

"Within the community they have a lot of solidarity and loyalty, this is one of Germany's great problems," said a senior German intelligence official, speaking on condition of anonymity. "There is no support within the communities to help German authorities find radicals."

Though evidence of the suspected forgery network like faked passports and visas was found at locations in Duesseldorf, Freiburg and Frankfurt, "our investigation over the last year against this circle centered on Ulm-Neu Ulm and the Multicultural House," Hoffmann said in a telephone interview from Munich. "It is basically a meeting point."

Investigators' early focus on those connected with the Multicultural House centered on a Sudanese doctor who was suspected of links to extremists but against whom there was never enough evidence to bring charges.

Officials said the doctor met in 1998 in the Ulm area with Mamdouh Mahmud Salim, bin Laden's suspected finance chief, only days before Salim's arrest by German authorities on a U.S. warrant. One witness told investigators the doctor had met with Mohamed Atta, lead suicide pilot in the Sept. 11 attacks, but that account has not been confirmed.

Salim, 46, was deported and is now in custody in New York awaiting trial on conspiracy charges in the 1998 bombings of two U.S. embassies in Africa, which killed 224 people. He was sentenced to 32 years in prison last year for stabbing a prison guard in the eye with a sharpened comb.

His case put authorities on to Syrian-born Mamoun Darkazanli, a Hamburg-based businessman who had power of attorney over one of Salim's bank accounts. Darkazanli has been jailed by German authorities on a Spanish warrant and is fighting extradition on charges he provided Al Qaeda logistical help. He denies the charges.

Recent focus has been on Egyptian physician Yahia Yousif, who was a target of the Jan. 12 raids but eluded police, Hoffmann said. It is possible Yousif fled after the December arrest of his son, Omar Yousif, on accusations he trained at a military camp of the Lashkar-e-Tayyaba group in Pakistan, which claims to fight Indian forces in Kashmir.