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A German extremist with links to Sept. 11 ringleader Mohamed Atta and other attackers has been detained in Syria, a U.S. official confirmed Thursday.

Mohamed Atta. AP file / AP

"Mohammad Haydar Zammar, a Syrian-born German national, was captured more than a month ago," Pentagon spokesman Eric Pahon told NBC News.

"This terrorist was captured during a unilateral operation by the Syrian Democratic Forces," he added, referring to the group of U.S.-backed militias fighting ISIS.

Zammar's detention was first reported by the AFP news agency.

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According to the congressional report into the 9/11 attacks by al Qaeda, Zammar had lived in the Germany city of Hamburg and was an "outspoken, flamboyant Islamist" and a "possible recruiter" of some 9/11 attackers.

The 9/11 Commission Report said Zammar was "a well-known figure in the Muslim community (and to German and U.S. intelligence agencies by the late 1990s)," adding that he had fought in Afghanistan and "relished any opportunity to extol the virtues of violent jihad.”

Atta was born in Egypt and studied in Hamburg. Atta was the head of the so-called Hamburg cell, which was central to the attacks on the United States.

After 9/11, “Zammar reportedly took credit for influencing Ramzi Binalshibh,” as well as to “the rest of the Hamburg group,” the congressional report added. Binalshibh was later sent to Guantánamo Bay for his alleged role in planning and providing logistical support for the Sept. 11 attacks.

The congressional report said that “owing to Zammar’s persuasion or some other source of inspiration,” by the late 1990s Binalshibh, Atta and fellow attackers Marwan al-Shehhi and Ziad Jarrah “eventually prepared themselves to translate their extremists beliefs into action.”

Binalshibh, Atta, al-Shehhi and Jarrah are considered part of the Hamburg cell, which “shared the anti-U.S. fervor” of other extremists, according to the congressional report, with the “added enormous advantages of fluency in English and familiarity with life in the West.”

Ramzi Binalshibh, fourth from right in the back row, and Mohamed Atta, seen kneeling in front of the man wearing the brown jacket giving the thumbs-up and with his hands on the shoulders of the person in the next row, visit the Al Quds Mosque in Hamburg, Germany, in 1999. DDP / AFP/Getty Images file

Atta, who is considered the operational leader of the 9/11 conspiracy, served as the pilot for American Airlines Flight 11 that crashed into the North Tower of the World Trade Center. Al-Shehhi flew United Airlines Flight 175 into the South Tower. Jarrah was flying United Airlines Flight 93, intending to crash it into either the Capitol or the White House in Washington, when it plowed into a field in Shanksville, Pennsylvania, following a revolt by passengers.

Zammar was detained by the CIA in Morocco in late 2001 and was later handed over to the Syrian government, Germany's Der Spiegel reported in 2005. At the time, the magazine said Zammar was being held in the notorious Far-Filastin prison in Damascus.

In 2007, a Syrian court sentenced Zammar to 12 years in prison for being a member of the Muslim Brotherhood, but he got out in 2013 after that country's civil war broke out, the AFP press agency reported.

According to German newspaper reports, he was released as part of a prisoner exchange between Islamist rebels and the government of President Bashar al-Assad of Syria. Zammar is believed to be in his 50s.