Abu Zubaydah is believed to have been the chief recruiter of the al-Qaeda network.
Zubaydah is accused of building up al-Qaeda in Pakistan
Former White House spokesman Ari Fleischer described him as a "key terrorist recruiter and operational planner and member of Osama Bin Laden's inner circle".
He was captured in Pakistan in 2002 and has been sent to Guantanamo Bay, Cuba.
The Palestinian, born in Saudi Arabia, is also known as Zayn al-Abidin Muhammad Husain and Abd Al-Hadi Al-Wahab.
He had strong connections with Jordanian and Palestinian groups and was sentenced to death in absentia by a Jordanian court for his role in a thwarted plot to bomb hotels there during millennium celebrations.
During the 1990s he was al-Qaeda's chief recruiter, selecting new members and arranging their visits to training camps.
"He receives young men from all countries. He accepts you or rejects you. And he takes care of the expenses of the camps," said Ahmed Ressam, an Algerian witness, during the trial of a man accused of the "millennium plot" at Los Angeles International airport.
US officials allege that Abu Zubaydah was behind that attack.
Master of disguise
Since the death of the al-Qaeda commander of military operations, Muhammad Atef, in a bomb attack in Afghanistan, he moved up the organisation's hierarchy.
Richard Reid, the so-called shoe bomber was arrested in 2001
He is alleged to have been heading the reorganisation of al-Qaeda in Pakistan, as well as planning new attacks on US interests.
He has alleged connections to a plan to blow up the US Embassy in Sarajevo, as well as a planned attack on the American embassy in Paris.
He is also thought to have briefed Richard Reid, the so-called shoe bomber arrested on board a flight from Paris to Miami in December 2001.
Abu Zubaydah was an elusive suspect. Few photographs of him were in existence, he had used at least 37 aliases and was considered a master of disguise.