Radical Islamists from a shut down Hamburg mosque linked to the September 11 attacks on the United States are now trying to infiltrate other mosques in and around the German city, according to officials and Muslim leaders. 

Small groups of radicals have turned up at several mosques trying to establish a new meeting place since the Taiba Mosque, where the 9/11 leader Mohammad Atta once prayed, was raided and closed by police in August, they told Reuters.

With radicals no longer grouped around one mosque near the city's main train station, security services have stepped up their observation of Islamists around the city and Muslim associations are on the lookout for suspicious newcomers.

"There are small groups of 3 to 5 people from the former Taiba Mosque who have gone to other mosques," said Ralf Kunz, internal affairs spokesman for the city government.

Individual radicals have turned up "at lots of mosques, both in Hamburg and the surrounding region," he said. "They have not been able to assemble at any mosque."

Hamburg police shut down the Taiba Mosque, formerly known as the Al Quds Mosque, in August, saying it had links with armed Islamist groups in Pakistan and Afghanistan.

A so-called "Hamburg Travel Group" of 11 radicals linked to the mosque left Germany in March 2009 to fight against the U.S. and coalition forces in Afghanistan.