U.S. Defense Secretary Donald Rumsfeld says he believes the most senior al-Qaida member in U.S. custody is in his words "a fountain of knowledge" concerning terrorist activities.
However, Abu Zubaydah has been too ill to talk much to investigators. He is recovering from gunshot wounds he received last month during his capture in Pakistan.
Pakistani officials are quoted saying he denied knowledge of who was behind the September 11 suicide hijackings in the United States during an initial interrogation on Thursday.
U.S. officials have refused to say where the suspected al-Qaida recruiter is being held.
Meanwhile, the alleged 20th September hijacker who was in jail at the time of the attacks has filed a lawsuit over his jailing conditions in the state of Virginia as he prepares for a U.S. federal criminal trial later this year.
Lawyers for French national Zacarias Moussaoui say he wants a larger jail cell, a computer and more telephone access to prepare for his trial on four conspiracy charges linking him to the plotting of the attacks. He faces the death penalty.
In other developments, the U.S. Defense Department says it has found pictures of U.S. Special Forces troops posing with a blindfolded and shackled John Walker Lindh, the American Taleban who was captured in Afghanistan.
His lawyers allege such pictures show their client was being tortured and that because of this comments he made at the time should be thrown out. The Pentagon has denied he received ill-treatment. The California native is charged with conspiring to kill Americans.