Mohammed Haydar Zammar leaves a mosque in Germany on Oct. 3, 2001. (Knut Mueller/Der Spiegel via AP)

A key figure in the radicalization of members of the Hamburg cell at the center of the Sept. 11, 2001, terrorist attacks has been freed from a Syrian prison where he spent years after being secretly sent to Damascus with U.S. backing.

A U.S. official confirmed that Mohammed Haydar Zammar is no longer in Syrian custody but didn’t know the circumstances behind his release. According to reports in Germany, which cited Zammar’s family, he was released last year as part of a prisoner exchange between rebel forces and the government of Syrian President Bashar al-Assad.

The exchange in the city of Aleppo, which has been the location of fierce fighting, involved swapping a number of political prisoners for Syrian army officers. The Ahrar al-Sham brigade, a part of the Islamic Front that has distanced itself from groups directly affiliated with al-Qaeda, helped arrange the exchange.

Zammar, once a well-known figure in Hamburg’s Islamist circles for his enormous girth and fiery views, helped facilitate the travel to Afghanistan of lead Sept. 11 hijacker Mohamed Atta and some of his cohorts.

He “relished any opportunity to extol the virtues of violent jihad,” according to the 9/11 Commission Report, which said Zammar “convinced” the members of the Hamburg group that they should go to Afghanistan.

Zammar, a German citizen of Syrian origin who left Germany in October 2001, was picked up in Morocco, interrogated by Moroccan and U.S. officials for a couple of weeks and then shipped to Damascus, German and Arab officials told The Washington Post in 2002.

At the time, Assad’s government was trying to win favor with the George W. Bush administration by positioning itself as an ally in the war against ­al-Qaeda.

There’s no indication that Zammar was aware of the plot to attack the United States. A key figure in the conspiracy, Ramzi Binalshibh, said under questioning that before returning to Germany from Afghanistan he was told to keep his distance from “conspicuous extremists” such as Zammar so as not to draw attention, according to the 9/11 report.

A native of Aleppo, Zammar has remained in his home city since his release, according to the reports by the German broadcaster NDR and the newspaper Süddeutsche Zeitung.

Zammar was a regular at Atta’s apartment and a radical mosque in Hamburg where he spoke of his time at an al-Qaeda terrorist camp in Afghanistan, according to German officials. Zammar was questioned immediately after the Sept. 11 attacks, but German officials said they didn’t have enough evidence to hold him. He left the country on Oct. 25, 2001.

Zammar was held for years without charge in one of Syria’s most notorious prisons before he was given a 12-year sentence for membership in the banned Muslim Brotherhood.