Isam Mohammed Taher al-Barqawi, also known as Sheik Abu-Mohammed al-Maqdisi, was arrested in 2005 and international human rights groups had called on authorities to release him or put him on trial.
He had shared a cell block between 1995 and 1999 with Abu Musab al-Zarqawi, who led al-Qaida in Iraq before being killed in a U.S. airstrike in 2006.
Al-Maqdisi, a native of the West Bank town of Nablus, is said to have taught radical Islamic ideology to the future al-Qaida commander. But hours before his arrest in 2005, he preached restraint to Iraq's insurgents, saying that he preferred a reduction in suicide attacks.
The call was rebuked by al-Zarqawi, who has warned in a statement posted on an Islamic Web site, that his mentor's comments could split Islamic fighters.