Mohammad al-Qahtani was a suspect in the first capital case at Guantanamo
The Pentagon has dropped charges against a Saudi citizen alleged to have been the "20th hijacker" in the 11 September 2001 attacks on the US.
Mohammad al-Qahtani was one of six Guantanamo Bay inmates charged with murder and war crimes in February.
The Pentagon said the case against the other five defendants would proceed.
Prosecutors are seeking the death penalty against the suspects in a case before military tribunals at the US prison camp in Guantanamo Bay.
A Pentagon official said the charges against Mohammad al-Qahtani had been dropped "without prejudice", meaning they could be reinstated.
The US military gave no reason for its decision.
But lawyers for the defendant say they believe the charges were dropped because he "was tortured" under interrogation.
The decision could have implications for the other five suspects, whose lawyers claim that similar treatment was meted out to them, the BBC's Adam Brookes reports from Washington.
Authorities say Mr Qahtani failed to take part in the 9/11 attacks because he was denied entry into the US by an immigration official.
He was refused entry at Orlando in Florida in August 2001 and returned to Dubai. He was later detained in Afghanistan and transferred to Guantanamo Bay.
In 2006, he recanted accusations he had made against fellow detainees of having links to al-Qaeda.
His lawyer told Time magazine the statements had been extracted under torture.
The Saudi was reportedly submitted to stress positions, sleep deprivation and humiliation at Guantanamo.
Officials said he had been subjected to a harsh interrogation authorised by former US Defence Secretary Donald Rumsfeld.
Four planes were hijacked by 19 men in the 11 September attacks. Two hit the World Trade Center in New York, another the Pentagon in Washington and the fourth crashed in Pennsylvania.
About 3,000 people were killed.
The five suspects still facing trial at Guantanamo include the alleged mastermind of the plot, Khalid Sheikh Mohammed.