Ahmad Wali Siddiqui looked relaxed as his trial opened at a court in the western city of Koblenz, chatting with his attorneys before beginning a lengthy statement to the court.
No pleas are entered under the German system and Siddiqui did not immediately address the charges against him, telling the court about his upbringing and how he immigrated to Germany as a teenager.
The 37-year-old was captured by U.S. troops in Afghanistan in July 2010 and while in custody provided details on alleged al Qaeda plots supposedly targeting European cities. No attacks materialized.
He is accused of membership in al Qaeda and the Islamic Movement of Uzbekistan and faces a possible 10 years in prison if convicted.
Prosecutors allege that Siddiqui trained with both terrorist groups in Pakistan and in the border region with Afghanistan in 2009 and 2010, with the aim of taking part in violent jihad, or holy war.
Authorities have said he was one of about a dozen radical Muslims who left the northern German port city of Hamburg in 2009 to pursue terrorist training in the border region. Several of them have been captured or killed.
Another member of the group, German-Syrian dual national Rami Makanesi, was convicted last year in a Frankfurt state court of membership in al Qaeda and sentenced to four years and nine months. He was arrested in Pakistan in June 2010 and then extradited to Germany.
Before going to Pakistan, Siddiqui and several other suspects met at Hamburg's al-Quds mosque, the prayer house that had served as a gathering point for some of the Sept. 11 attackers before they moved to the U.S. to attend flight schools in 2000, German intelligence officials have said. The mosque has now been shut down by authorities.
Intelligence officials also said Siddiqui was a friend of Mounir el Motassadeq, who was convicted by a German court in 2006 of being an accessory to the murder of the 246 passengers and crew on the four jetliners used in the Sept. 11 terrorist attacks. El Motassadeq also frequented the al-Quds mosque.
"He wasn't my friend," Siddiqui told the Koblenz court. He said they first met when they both worked for the same aviation services company for several months.
Despite distancing himself from El Motassadeq, Siddiqui acknowledged that he drove El Motassadeq's father some 400 kilometres to western German Wuppertal in 2002 to visit his son in a prison there.
Siddiqui also said he did not personally know Sept. 11 plotter Ramzi Binalshibh and suicide hijacker Mohamed Atta, who had both lived in Hamburg before the attacks.
Prosecutors maintain Siddiqui received general military training at a camp run by the Islamic Movement of Uzbekistan and helped produce a German-language propaganda film.
In the summer of 2009, he decided to leave the group's camp, and moved to an al Qaeda training area where he learned how to use heavy weapons, including anti-tank weapons and mortars, prosecutors said.
In June 2010, a "high-ranking al Qaeda member" instructed Siddiqui to return to Germany to become part of a European network of the terrorist organization, prosecutors said in a statement when Siddiqui was charged.
"The network was supposed to secure financial support for the organization, but at the same time be ready for other, not yet concrete, orders from the al Qaeda leadership," the statement said.
German magazine Der Spiegel, which obtained the full 114-page indictment, identified the al Qaeda leader behind the orders as Younis al-Mauritani, who was apprehended in 2011 by Pakistani agents working with the CIA.
After receiving his orders, prosecutors said Siddiqui slipped across the border into Afghanistan to return from there to Germany, but was captured by American troops in Kabul before he could leave the country.
Intelligence officials have said that, while in American custody, he provided interrogators with details of an early stage terrorist plot in Europe around Christmas 2010, which led the U.S. and others to issue a travel alert for Europe.
He was turned over to German authorities last April and is being tried in Koblenz, because it is near where he was brought back in to Germany at the U.S. Air Force's Ramstein Air Base.
Siddiqui has seven sisters and brothers, one of whom also accompanied him to Pakistan but returned quickly. In addition, Siddiqui has a half sister who lives in the U.S. from his father's first marriage to an American woman.