On Marien Street, a Hamburg thoroughfare lined with apartment buildings with red-tiled roofs, No. 54 is an unremarkable off-white house.
In a country that is home to about 7 million immigrants, the sight of several clean-cut Arab students coming and going from the lace-curtained home is not unusual.
This was the home of Mohamed Atta, the presumed ringleader of the Sept. 11 attacks who apparently piloted one of the jets into the World Trade Center. Atta shared the home with Marwan al-Shehhi, suspected of crashing the second plane in New York, and Ziad Jarrah, suspected of flying the plane that crashed in Pennsylvania.
In hindsight, authorities are learning lessons about how they could have detected the presence of a terrorist cell. But one of the biggest needs, they say, is for better cooperation among agencies - cooperation that may have linked Atta's cell with operatives now under arrest in Madrid.
Atta was believed to have been in contact with Imad Eddin Barakat Yarbas, also know as Abu Dahdah, head of a Spanish Islamic group that called itself "The Soldiers of Allah." Mr. Dahdah reportedly traveled widely, contacting other Islamic extremists in Indonesia, Malaysia, and Jordan.
The Hamburg cell's cooperation with the one in Madrid shows how easy it is for Al Qaeda groups to blend into countries with large immigrant populations. "We had no suspicions that there was an Al Qaeda cell in Hamburg," a high-ranking German secret service official says. "We knew there were some Islamic extremists in the city, but these young men were totally unsuspicious. They didn't so much as get a parking ticket."
German and American law enforcement agencies are also searching for three fugitives: Said Bahaji of Germany, Zakariya Essabar of Morocco, and Ramzi bin al-Shibh of Yemen.
Analysts speculate that, although militants involved in the Sept. 11 attacks lived in many European countries, they were particularly attracted to Germany because of its lax immigration laws and population of 3 million Muslims.
Hamburg, in particular, has a large, well-integrated Muslim community, including the largest Afghan community in Europe. Because Hamburg's Muslim community was quiet and moderate, it was also not under heavy surveillance.
Out of 122 secret service agents in Hamburg, only one was assigned to "Islamic extremists," which include everything from the ethnic-Albanian UCK to the Turkish PKK. Until Sept. 11, Hamburg security forces were more concerned with the city's left-wing extremists who periodically engage in destructive demonstrations. …