Abu Zubaydah was held up during the Bush administration as one of al-Qaeda's senior members and part of Osama bin Laden's inner circle.
He himself has denied a connection with al-Qaeda, but he has certainly supported its aims.
In a recruitment video for his own terror group, the Martyrs Organization, he praised the attacks on the U.S. on Sept. 11, 2001 as "truly magnificent." After the attacks, he wrote, "We were in a state of elation that God only knows."
From 1990 until his capture in 2002, he chronicled his life in diaries that were found by the U.S. government. Those six notebooks were used to justify his ongoing detention as well as that of other prisoners at Guantanamo.
They also provide a unique first-person account of the time period when Islamist militant groups became important players on the world stage, while giving a glimpse into the inner life of someone joining a jihadist movement.
CBC Radio's Day 6 host Brent Bambury talks to Miami Herald journalist Carol Rosenberg about the importance of Zubaydah's writings.
In addition, Toronto actor Yasin Sheikh reads portions of the first volume of the diaries for Day 6.
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