Six Guantanamo Bay detainees are challenging US federal authorities to reveal evidence of abuse at the camp.
Families of the six men have been calling for their release
The six - all of Algerian origin and extradited from Bosnia - are suing the Bush administration as part of their effort to contest their detention.
They want the government to release documents which they say would prove that prisoners were tortured.
In their legal action, they say one of the six was beaten so badly by jailers that he suffered facial paralysis.
The detainee, Mustafa Ait Idir, alleges that his hands were tied behind his back while the beating took place.
"The guards picked him up and slammed his body and his head into the steel bunk in his cell," according to legal papers filed on Wednesday at a court in Boston.
The guards then pushed his face into a toilet and "repeatedly pressed the flush button", the account continued.
It said the detainee suffered a stroke after the beating, leaving half of his face paralysed.
Pentagon spokesman Maj Michael Shavers said the department did not discuss specific cases, but added that it was US policy to treat detainees humanely.
The six were arrested in late 2001 in Bosnia on suspicion of plotting attacks against the US and UK embassies in Sarajevo.
Despite a ruling by Bosnia's highest court that they should be released, the men were handed over to the US in 2002 and have been held as "enemy combatants" ever since.
Their legal action is aimed at forcing the US authorities to release documents, medical records and videotapes regarding their detention.
Lawyers for the men say that despite numerous requests under the Freedom of Information Act, they have been unable to obtain any of the documents they seek.
In January, more than 100 people staged a protest in Sarajevo demanding freedom for the men.
Volunteers from different Muslim countries helped Bosnian Muslims during the bloody war against Serbs and Croats in the early 1990s. Many of them chose to stay and became Bosnian nationals.
The legal action comes as terror suspect Jose Padilla, held without charge in a US military prison since 2002 for allegedly trying to set off a "dirty bomb", enters a new phase in his efforts to secure his freedom.
Mr Padilla was due to be released on Thursday, after a US federal jduge ruled that his continued detention was unlawful and gave the Bush administration 45 days to let him go.
However, the order is now apparently on hold while federal prosecutors file an appeal, correspondents say.