Five alleged co-conspirators in the September 11 attacks have been referred for military trial on capital charges, but charges against a sixth -- the alleged "20th hijacker" -- were dropped without explanation, the Pentagon said Tuesday.

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

Pentagon spokesman Bryan Whitman said Crawford gave no reason for dropping the charges.

Qahtani remains imprisoned at Guantanamo Bay, Cuba. The ruling does not affect his status as an enemy combatant, official said. Whitman said the action also 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.

Qahtani had been charged with the killing of 2,973 people in the September 11, 2001 attacks in New York and Washington, along with five others, including Khalid Sheikh Mohammed, the alleged mastermind.

Crawford ordered Mohammed and the other four to stand trial together before a special military commission in Guantanamo Bay Cuba on war crimes charges that carry the death penalty.

"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.

The other alleged co-conspirators are Walid Muhammad Salih Mubarak Bin Attash, Ramzi Binalshibh, Ali Abdl Aziz Ali, and Mustafa Ahmed Adam al Hawsawi.

The five will be arraigned as early as June 12.

The charges allege that Khalid Sheikh 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.

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 training camp in Afghanistan where two of the hijackers were trained, and traveled to Malaysia in 1999 to observe 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 dollars for expenses and flight training, and to have arranged travel to the United States for nine of the hijackers.

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 September 11.

Qahtani is alleged to have traveled to the United States in August 2001 to join the 19 other hijackers who flew commandeered airliners into the World Trade Center, the Pentagon, and a field in Pennsylvania.

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 authorized by then-Defense Secretary Donald Rumsfeld.

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