Majid Khan,42, was moved to Belize on Thursday after spending 16 years in CIA custody. Authorities say he helped deliver money and transport other senior al Qaeda officials.
Prosecutors are in talks with attorneys for Khalid Shaikh Mohammed and his four co-defendants about a plea agreement that could eliminate the prospect of the death penalty.
The Pentagon is building a second secret court oom at Guantanamo Bay to hold trials for suspected terrorists at the same time, as calls have grown for President Biden to shut down the prison.
The interagency Periodic Review Board approved the transfer of Sanad Yislam al-Kazimi and Assadullah Haroon Gul, bringing the number of detainees at Guantanamo down to 37.