The jihadist is known for allegedly helping plan the September 11, 2001, attacks within the US, including recruiting a few of the hijackers to the terror group.
United States officials have said that there are hundreds of foreign fighters and thousands of Syrian militants who belong to Daesh in SDF custody. According to reports in Germany, which cited Zammar's family, he was released past year as part of a prisoner exchange between rebel forces and the government of Syrian President Bashar al-Assad.
Al-Qaeda operated a branch in Syria known as Al-Nusra Front, but the affiliate has since claimed to have broken off ties.
Zammar has been suspected by the USA authorities to have recruited Mohammed Atta, an Egyptian national who was one of the pilots involved in the attacks.
Zammar, a German citizen of Syrian origin who left Germany in October 2001, was picked up in Morocco, interrogated by Moroccan and US officials for a couple of weeks and then shipped to Damascus, German and Arab officials told The Washington Post in 2002.
The report also claimed he "fought in Afghanistan and relished any opportunity to extol the virtues of violent jihad" and took credit for influencing Binalshibh, 9/11 ringleader Mohammed Atta and others in the "Hamburg Group" following the attack.
Zammar was mentioned several times in the 9/11 Commission's report, which said he encouraged worshippers at a Hamburg mosque towards terrorism.
A German gentleman suspected of recruiting a few of those 9/11 hijackers to alqaeda has been arrested by forces in Syria, the Pentagon said Thursday. He left the country on October 25, 2001.
According to the AFP, a Syrian court sentenced Zammar to 12 years in prison in 2007 for being a member of the Muslim Brotherhood, but he was released in 2013 after the Syrian civil war broke out as part of a prisoner exchange between Islamist rebels and the President Bashar al-Assad's government.