Mohammed Haydar Zammar was an outspoken cleric in Germany. He's accused of helping to arrange 9/11 head hijacker Mohammed Atta's trip to Afghanistan to train with al-Qaeda. German officials say the two frequently met with each other.
The US 9/11 Commission Report, issued in 2004, said Zammar "relished any opportunity to extol the virtues of violent jihad."
Zammar was detained in Morocco in a CIA operation just months after the attack that destroyed the World Trade Center in New York City and damaged the Pentagon. He was handed over to the Syrian government two weeks later. In 2007, Zammar was sentenced to prison by the Syrian government for 12 years for being a member of the Muslim Brotherhood. It is believed that during his prison sentence, the US government would submit written questions for the Syrian government to pose to Zammar, which were then relayed back to the Americans.
In 2013, Zammar was released as part of a prisoner exchange deal between the Assad government and the rebel groups attempting to depose him. The government released 2,130 prisoners in exchange for 48 Iranians held captive by rebels. The group Ahrar al-Sham brokered the deal, which saw Zammar's release back to his birth city of Aleppo.
— Björn Stritzel (@bjoernstritzel) April 17, 2018
Years later, when the government liberated Aleppo from a laundry list of jihadist groups, Ahrar al-Sham members executed hundreds of Syrian Army hostages before fleeing the city.
It isn't clear if Zammar knew about the September 11 attacks before they happened, although the CIA claims he did. Accounts from Ramzi bin al-Shibh, a captive at Guantanamo Bay, suggest otherwise; however, Zammar knew Osama bin Laden personally. Zammar began his terrorist career in the 1990s during the Soviet-Afghan war. Once a member of al-Qaeda, Zammar joined Daesh in Aleppo after his release from prison in 2013, accusing Ahrar al-Sham of "treachery." Kurdish officials indicate that Zammar is currently being interrogated by the YPG, a Kurdish militia that comprises about 60 percent of the US-backed Syrian Democratic Forces (SDF).