Eavesdropping Thwarts 9/11 Anniversary Terror Attack

Officials say Mr. Gelowicz returned to Germany, where he led the would-be terrorist cell that planned a series of enormous car bombs intended for targets in his country where there were likely to be many Americans. It is difficult to say how Mr. Gelowicz, after so much police attention, thought he could succeed.

But he was nothing if not determined. “He was possessed with the desire to launch an attack,” said Mr. Hanning, who added that Mr. Gelowicz was among the roughly 100 to 150 Islamists considered dangerous by the German government and kept under regular surveillance.

The cell tried to avoid detection, investigators said, by using public telephones and Internet cafes rather than their own phones and computers. Instead of sending messages that could be intercepted, they left one another messages saved in the draft folders of e-mail accounts for which multiple members had passwords.

For all their preparations, investigators said, the plot did not succeed. Hundreds of investigators from several jurisdictions and agencies kept them under close watch for several months, and officials said they even swapped diluted hydrogen peroxide for the bomb-ready concentrated solution Mr. Gelowicz’s cell was preparing to use.