The US has referred five accused co-conspirators in the 11 September attacks for military trial but dropped charges against the alleged "20th hijacker" without explanation, the Pentagon said on Tuesday.

Khalid Sheikh Mohammed, the alleged mastermind of the 11 September attacks, and four others were referred for trial by a special military commission on capital charges of murder, terrorism and other war crimes, it said.

But Susan Crawford, the convening authority for the war crimes tribunals, "dismissed without prejudice" charges against Mohammed al-Qahtani, a Saudi who was subjected to harsh interrogations at Guantanamo Bay, Cuba.

Crawford gave no reason for dropping the charges, Pentagon spokesperson Bryan Whitman said. However, he said the action does not prevent military prosecutors from refiling war crimes charges against Qahtani at a later date.

"What that means is that the government retains the option of charging Qahtani separately," he said.

Remains imprisoned

Qahtani, who has been held at Guantanamo Bay, Cuba, for more than six years as an enemy combatant, will remain imprisoned despite the charges being dismissed.

Qahtani and the other five men were charged in February with the killing of 2973 people in the 11 September 2001 attacks, in which 19 men commandeered airliners and crashed them into the World Trade Center in New York and the Pentagon in Washington, and a field in Pennsylvania.

"The charges allege a long-term, highly sophisticated plan by al-Qaeda to attack the United States," said Whitman.

"Each of the five... are charged with conspiracy, murder in violation of the law, attacking civilians, attacking civilian objects, causing serious bodily injury, destruction of property in violation of the law of war, terrorism and providing material support to terrorism," he said.

Possible death sentences

Now facing trial and possible death sentences with Khalid Sheikh Mohammed are Walid Muhammad Salih Mubarak Bin Attash, Ramzi Binalshibh, Ali Abdl Aziz Ali, and Mustafa Ahmed Adam al Hawsawi.

They will be arraigned as early as 12 June, and the trial itself is supposed to be held within 120 days of the defence counsel being notified of the referral.

The American Civil Liberties Union, meanwhile, charged that the Pentagon was delaying security clearances for two lawyers who have volunteered to represent Khalid Sheikh Mohammed.

Failure to grant the clearances before Mohammed speaks publicly in court "will once again demonstrate how these proceedings are fundamentally unfair and bordering on a farce," said Anthony Romero, the ACLU's executive director.

The charges allege that Mohammed proposed the operational concept for the September 11 attacks to al-Qaeda leader Osama bin Laden as early as 1996, according to the Pentagon.

Actions of the co-conspirators

After receiving Bin Laden's approval and funding, Mohammed oversaw the operation and trained the hijackers in Afghanistan and Pakistan, it said.

Bin Attash allegedly administered a camp in Afghanistan where two of the hijackers were trained, and traveled to Malaysia in 1999 to observe US airline security in preparation for the attack.

Binalshibh allegedly lived with three of the hijackers in Hamburg, Germany.

Denied a visa to the United States, he allegedly helped find flight schools for the hijackers and engaged in financial transactions as part of the plot, according to the Pentagon.

Aziz Ali is alleged to have sent hijackers $120 000 for expenses and flight training, and to have arranged travel to the United States for nine of the them.

Al-Hawsawi is alleged to have provided hijackers with money, travelers checks, credit cards and western clothes, and transferred money between his accounts and theirs on 11 September.

Qahtani is alleged to have travelled to the United States in August 2001 to join the 19 other hijackers.

But he was stopped at Orlando's International Airport where immigration authorities denied him entry and put him back on a flight to Dubai.

The United States has alleged that Mohammed Atta, the lead hijacker, had driven to Orlando meet Qahtani.

Qahtani was later captured in Afghanistan and taken to the US military detention center at Guantanamo Bay, Cuba.

In late 2002 he was subjected to a special interrogation regime authorised by then-Defence Secretary Donald Rumsfeld.

He reportedly was subjected to sleep deprivation, loud music and extremes of temperature during interrogations lasting 20 hours a day.

AFP