Berlin, Germany (CNN) -- In a quiet leafy suburb of Hamburg there lives a 51-year-old German of Syrian descent named Mamoun Darkazanli. He is a businessman and a preacher, and he is of great interest to the city's intelligence agencies.
Until recently Darkazanli was the imam of the Taiba mosque in the city, a mosque previously known as the Al Quds mosque.
It was there in the late 1990s that he became closely acquainted with Mohammed Atta, the lead 9/11 hijacker, and at least one other of the 9/11 conspirators. In the past he has described those relationships as coincidences.
The 9/11 Commission Report linked Darkazanli to al Qaeda fund-raising.
Other Islamists have gravitated to his mosque and his preaching in recent years, including several of a group that left Germany in March 2009 to wage jihad, or holy war, on the Afghan-Pakistan border region.
European intelligence officials say some of them later became involved in a plot linked to al Qaeda's plans to launch "Mumbai-style" terror attacks in Europe.
The Taiba mosque was finally closed in August after a long legal battle. Darkazanli faces no charges in connection with its activities or members, and an attempt by Spain to extradite him -- because of alleged contacts with al Qaeda members there -- failed.
But in an interview with CNN, Hamburg's intelligence chief Manfred Murck expressed his frustration that Darkanzali was still free to preach his radical messages.
"We knew him even before 911... we still believe that he was, and maybe still is, a kind of representative of al Qaeda in Hamburg," Murck told CNN.
German intelligence officials believe Darkanzali, despite being very careful with the wording of his sermons at the Taiba mosque, helped inspire the group that traveled to Pakistan in March 2009 to fight jihad.
Darkanzali was closely acquainted with Naamen Meziche and Ahmed Sidiqi, two of the leading figures in the group, according to German intelligence officials. Sidiqi is currently being held in Bagram airforce base, where he has revealed details about the plot against Europe according to counter-terrorism officials.
"When it comes to the last speeches [Darkanzali] gave then he told them 'Allah help to kill our enemies...' so it was very general, it was not 'Let's kill that one, or destroy that city.' It was more a general cry for help to Allah to help the brothers against the enemies but it was not enough for our police to open an investigation against him," Murck said.
Murck said the Taiba mosque acted as an incubator for young Muslim men searching for a sense of identity and purpose.
Two weeks after the mosque closed Darkazanli, speaking through the intercom at his apartment building, expressed outrage to CNN.
"To close down the Taiba mosque during Ramadan is an insult to all Muslims," he said. He claimed the move was a political stunt by Hamburg's new mayor.
After Sidiqi's arrest became public in September, CNN made several attempts to contact Darkazanli at his address in Hamburg, but received no reply.
Despite the closure of the Taiba mosque Hamburg intelligence officials are concerned that Darkazanli may try to found another mosque in the city, in which case authorities fear they may have to fight some of the same legal battles again.
Some of the radicals who formerly worshiped at the Taiba mosque have temporarily relocated to other mosques in the city, according to German intelligence officials.
As Hamburg authorities try to anticipate Darkanzali's next move, they are watching him closely.