Even so, the intelligence official in Hamburg said Germany did face the “intense abstract danger” of homegrown radical Islamic terrorism. And this week, the Interior Ministry also said it had assigned an additional 200 investigators to focus on terrorism.

German officials said that hundreds of their citizens and legal residents had over the years traveled to the lawless border region between Afghanistan and Pakistan, and that more than 100 had returned to Germany. There has been a deluge of statistics coming from Germany’s intelligence and police operations illustrating the growing concern of radicalized young people heading off to terrorist training camps  and then returning home to strike.

The newspaper Der Tagesspiegel reported that federal officials identified 131 individuals as possibly prepared to carry out violent acts. The federal security services said that over the last two decades, about 215 citizens or legal residents of Germany received or intended to receive paramilitary training, that 65 completed the training, and that of the total group, about 105 are in Germany, including 15 in prison.

Those precise-sounding numbers, however, belie the fact that German officials are struggling to get a handle on how susceptible their large and mostly peaceful population of Muslims is to the lure to radicals in far-off places.

“These are the so-called known terrorists, the suspicious people,” said Rolf Tophoven, the director of the Institute for Terrorism Research and Security Policy in Essen, of the statistics. “But the unknown people, they don’t go to the mosque to pray because they are afraid to be detected; they don’t have meetings in religious institutions, maybe they come together in a private house. That is a big concern.”

Germany first awoke to this reality in 2001, when three men who worshiped at a Hamburg mosque were identified as leaders in carrying out the Sept. 11 plot including Mr. Atta, who flew one of the planes into the World Trade Center.

Memories of that case resurfaced when the American military arrested Mr. Sidiqi, a German citizen of Afghan heritage who once worked at the Hamburg airport. The Hamburg intelligence official that said Mr. Sidiqi had been under surveillance for years and that like Mr. Atta, he had attended Al Quds Mosque in Hamburg. The official said Mr. Sidiqi was also a friend of Mounir el-Motassadeq, who was convicted in 2006 for his role in aiding the Sept. 11 hijackers.