But the US government said in the Manhattan federal court on Monday that gathering intelligence from Ahmed Khalfan Ghailani during interrogations was "weightier, more significant" than giving him a speedy trial.
The case of Ghailani, a Tanzanian national charged over his alleged role in the 1998 US embassy bombings in Tanzania and Kenya, is being watched for precedents that could affect other Guantanamo detainees.
Khalid Sheikh Mohammed, the accused mastermind of the September 11, 2001 attacks in the US, is also due to be tried in Manhattan federal court.
Alluding to the importance of the proceeding, US District Judge Lewis Kaplan said during Monday's session: "I think everybody can agree that whatever I do here would be unprecedented."
Ghailani was taken into custody in Pakistan in July 2004 and interrogated outside the US as part of the Bush administration's "extraordinary rendition" programme in which suspects were captured in one country and interrogated in another.
He was transferred to Guantanamo Bay in 2006 and his case was moved to the Manhattan federal court last June.
He has pleaded not guilty to charges including conspiring with Osama bin Laden and other members of al-Qaeda to kill Americans, as well as separate charges of murder for the 224 people killed in the African bombings.Ghailani's case, which coincides with the eighth anniversary of the first group of 20 detainees being sent to Guantanamo in 2002, is a test for Barack Obama's plans to shut down the US military prison.
Last week, the US president reiterated his commitment to shut it down although the White House has conceded that he will not meet his self-imposed deadline of the end of January.
Obama also suspended the transfer of Guantanamo detainees to Yemen last week, following revelations that Umar Farouk Abdulmutallab, the Nigerian man accused of trying to destroy a Detroit-bound US airliner on December 25, reportedly received al-Qaeda training in Yemen.
Nearly 200 detainees remain at Guantanamo, nearly half of them from Yemen.
The US government is working to refurbish a prison in the state of Illinois to hold some detainees while their fate is determined.