The Post writes that the Justice Department and the White House declined to comment on pending criminal charges against Marri.
The Associated Press reports that Marri was first arrested in 2001.
The government says al-Marri is an al-Qaida sleeper agent who has met Osama bin Laden and spent time at a terrorist training camp in Afghanistan.
A legal U.S. resident, al-Marri has been held in solitary confinement at the brig since 2003.
After he was arrested as a material witness in late 2001, authorities first charged him with credit card fraud. Later, officials said he had strong links to al-Qaida terrorists, so President Bush declared him an enemy combatant and transferred him to the military brig.