German officials said the Hamburg group members were recruited from the Taiba mosque in Hamburg. In the 1990s, that same mosque — then called Al Quds — was attended by Mohamed Atta, who went on to become the lead hijacker in the 9/11 attacks.

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A friend of Atta from those days has emerged as a crucial figure in the new plot, European intelligence officials tell CNN. Naamen Meziche, 40, a French citizen of Algerian descent, worked to persuade a number of young men praying at the Taiba mosque to join in jihad, the officials said. Though his exact whereabouts are unknown to authorities, he is thought to be in the Afghan/Pakistan border area. Meziche’s wife told CNN that he was overseas.

In the years after 9/11 the Taiba mosque became a magnet for al Qaeda sympathizers across Europe. “They all wanted to come and pray where Mohamed Atta prayed,” a German intelligence official told CNN.

Hamburg authorities shut the mosque a few weeks after Sidiqi was arrested. The decision to shut the mosque was difficult, said officials in Hamburg, because the presence in one place of so many militants made it easier to monitor their activities. But they said the mosque had become a recruiting center for jihadists across Europe.

Several militants now back in Germany who failed to make it to Pakistan’s tribal areas are of continuing concern to members of German intelligence services, who have kept them under observation.

“Their greatest enemy is the United States,” a German intelligence official told CNN.

A recent report by Hamburg’s intelligence services said that 45 jihadists lived freely and openly in the city, from where they supported al Qaeda. High evidence thresholds under the German legal system have made it very difficult for authorities to make arrests, German officials told CNN. In addition to those actively supporting al Qaeda, another 200 Islamists living in the city are described as having “violent tendencies.”