(CNN) -- An al Qaeda plot to attack European targets would have borne a chilling resemblance to the deadly 2008 assault by armed militants on the Indian city of Mumbai, Western intelligence sources have revealed.
Details of the planned operation reportedly came from a German citizen of Afghan descent, with links to a mosque in Germany known to have been the meeting place for those behind the September 11, 2001, attacks on the United States.
Who is this Afghan-German suspect?
According to an unnamed German counterterrorism official, Ahmed Sidiqi is from Hamburg in northern Germany. He worked for a cleaning company at the city's international airport until 2009, when he and several other Germans traveled to the Afghan-Pakistan border and joined the Islamic Movement of Uzbekistan -- an extremist group allied with al Qaeda.
What about the mosque he attended?
Sidiqi attended the Masjid Taiba mosque -- formerly known as the Al-Quds mosque -- in Hamburg. In 2001 it was identified as the meeting place of those behind the 9/11 attacks on New York and Washington.
Among those who prayed there was Mohammad Atta, one of the hijackers who commandeered the first airliner that crashed into New York's World Trade Center. Sidiqi was part of Atta's circle, according to the German intelligence official.
In August this year, Hamburg city authorities shut down the mosque, along with an adjacent Taiba cultural center. A statement from the Interior Senate of Hamburg read: "Recent events have again shown that instructional courses, sermons and seminars held by the organization and texts published on its website are not only aimed against constitutional regularity, but also seek to radicalize their listeners and readers."
The senate statement added that a group of individuals had left Germany "to support the armed conflict on the Afghanistan/Pakistan border after having previously attended Taiba events on a regular basis." One of them later appeared in a German-language video soliciting support for the so-called "Holy War," officials said.
"Hamburg must not serve as the incubator for Islamists willing to employ violence," Christoph Ahlhaus, the head of Hamburg's interior ministry, was quoted as saying.
When was Sidiqi captured?
Sidiqi was detained in Kabul in July of this year and transferred to U.S. custody where he revealed details about the terror plot while being questioned. "He started to talk a lot," and detailed a "Mumbai-style" attack in Europe, the German official told CNN. It remains unclear how significant a role he had, if any, in planning the proposed attacks.
Could this mark a change in al Qaeda tactics?
"The high-profile attacks that it has always liked using explosives are clearly getting harder and harder to perpetrate," said CNN's Senior International Correspondent Nic Robertson.
Since the September 11 attacks on New York and Washington, al Qaeda and its fugitive leadership have found it difficult to operate with their hideouts in Pakistan under increasing scrutiny and more cells around the world being uncovered.
However, intelligence experts say al Qaeda is resilient with the ability to diversify its operations. Last week, top U.S. security officials told a Senate committee the terrorism threat against the United States had evolved, with homegrown terrorists and a greater diversity in the scope and methods of attack making it more difficult to prevent them.
"Mumbai would have been viewed as successful by the al Qaeda leadership because it killed a large number of people," said Robertson. "This type of attack is just as deadly but harder to stop."
Nic Robertson and Paul Cruickshank contributed to this report.