Mr. Abu Ghaith, a 48-year-old Kuwaiti-born cleric known for his fiery oratory, had recorded impassioned speeches for Bin Laden after Sept. 11, in which he praised the attacks and promised that future attacks would be carried out.

The defendant unexpectedly took the witness stand last week, offering a vivid account of being summoned by Bin Laden on the night of the attacks to meet with him in his cave in the Afghan mountains.

“He said, ‘Come in, sit down.’ He said, ‘Did you learn about what happened?’ ” Mr. Abu Ghaith recalled the Qaeda leader telling him.

Mr. Abu Ghaith said Bin Laden claimed credit for the attacks and told him the next day that he wanted Mr. Abu Ghaith to help him “deliver a message to the world.”