By Adam Brookes
BBC News, Washington
Mohammad al-Qahtani was a suspect in the first capital case at Guantanamo
The American government has given no reason why charges against the man it has alleged was the "20th hijacker" in the 11 September 2001 attacks on the US have been dropped.
Mohammad al-Qahtani has been held at Guantanamo Bay since 2002, following his detention in Afghanistan.
In February, he was charged with conspiracy, terrorism, and murder in violation of the laws of war, among other offences.
The US alleges he attempted to come to the United States in order to take part in the 9/11 attacks, but was stopped at the airport on his arrival.
An immigration officer suspected he intended to stay in the US illegally, and refused him entry.
The charges were dropped "without prejudice" - which means they could be brought again at a later date.
Five other men were charged alongside Mr Qahtani.
They include Khaled Sheikh Mohammed - the man accused of organising the 9/11 attacks.
Their trials before military commissions - the special military courts in Guantanamo Bay - are slated to go ahead.
As well as his military lawyer, Mr Qahtani is represented by a civilian lawyer from the Center for Constitutional Rights - a New York-based legal rights organisation.
The CCR said in a statement it believed the charges against him had been dropped because Mr Qahtani had been tortured.
"The government's claims against our client were based on unreliable evidence obtained through torture at Guantanamo," it said.
The Guantanamo prison camp was set up soon after the Afghanistan invasion.
"Using torture to string together a web of so-called evidence is illegal, immoral and cannot be the basis for a fair trial."
Published reports in 2006 described Mr Qahtani's interrogation.
The reports - based on leaks from the Pentagon - said he had been subjected to stress positions, sleep deprivation, extreme temperatures, humiliation and other highly coercive practices.
Some lawyers believe military officers did not want to face a discussion of these interrogation techniques in court, nor to have their case collapse publicly because the evidence obtained using such techniques might be ruled inadmissible.
However, proceedings against the five other suspects, including Khaled Sheikh Mohammed, appear to be going ahead.
It has been frequently reported, and is widely believed by civilian and military lawyers, that similar interrogation techniques were used in these cases, too.
So how are they able to go ahead, if the case against Mr Qahtani is dropped?
Lawyers suggest that in those cases there may be other evidence - obtained independently, and not tainted with the threat of inadmissibility.
One lawyer who is not directly involved with the Guantanamo detainees called the failure of the case against Mr Qahtani a "huge setback" for the US government and the entire legal process at Guantanamo Bay.
"Yet again, we don't know what is really happening in this system," he said. "Transparency is zero."
The military commissions process remains extremely controversial.
A case before the Supreme Court - Boumediene vs Bush - challenges its very legality under the constitution. The Court is expected to rule in the next two months.