"We can confirm that Mohammad Haydar Zammar, a Syrian-born German national, was captured more than a month ago by SDF (Syrian Democratic Forces) partners as part of their ongoing operations to defeat ISIS inside Syria," Reuters quoted Pentagon spokesman Eric Pahon as saying. According to Middle East Eye, citing the AFP, Zammar has long been known to Western as well as Syrian authorities for his direct links to key al-Qaeda planners of 9/11:

A Syrian-born German national accused of helping to plan the September 11, 2001 attacks has been detained by Kurdish forces in Syria, a senior Kurdish commander told AFP on Wednesday. The jihadist is best known for allegedly helping plan the Sept. 11, 2001, attacks in the U.S., the report said, including recruiting some of the hijackers to the terror group. As the Pentagon noted, the official 9/11 Commission Report described him as an “outspoken, flamboyant Islamist” who extolled “the virtues of violent jihad.”

Zammar fled Germany after the attacks and relocated to Morocco, where he soon was arrested in an operation involving CIA agents. He later was handed to Syrian authorities who, in 2007, sentenced him to 12 years in prison for being a member of the Muslim Brotherhood, according to German state broadcaster Deutsche Welle. Zammar spent years in Syrian prison after the CIA captured him in Morocco and transferred him to Damascus as part of the CIA's "extraordinary rendition" program, however, in the midst of the war he was released by the Assad government as part of a general amnesty for political prisoners deal which at the time was demanded by the Syrian opposition and international human rights groups (though some prisoners broke free when insurgent groups took over government towns early in the war). Zammar is believed to have been an al-Qaida recruiter and close associate of members of the Hamburg terror cell of Islamic extremists thought to have been important operatives in the 9/11 attacks in the United States.
Mohammed Haydar Zammar, a recruiter for the September 11 attacks, is arrested in Syria.