German police have shut down the Hamburg mosque where the 9/11 hijackers met before their suicide attacks on the US in 2001.
About 20 police officers searched the Taiba mosque early on Monday; its associated cultural organisation is suspected of helping extremists.
The Taiba association has been banned.
Investigators suspect that members of the Islamic Movement of Uzbekistan met at the Taiba mosque before going to a jihadist training camp.
The mosque in Hamburg's St Georg district - formerly called the Al-Quds Mosque - has been under surveillance since the 2001 attacks.
German media say a recent intelligence report described the mosque as "a major hub for the jihadist scene in Hamburg", which it said grouped 45 radical Islamists.
US investigators say an al-Qaeda cell based in Hamburg masterminded the 9/11 attacks, in which hijackers seized four planes and crashed them into the World Trade Center in New York, the Pentagon in Washington, and a field in Pennsylvania.
About 3,000 people died in the attacks - the worst acts of terror carried out on US soil in modern times.
