Washington  - The Pentagon announced Tuesday that charges have been dropped against a man suspected of being the "20th hijacker" in the September 11, 2001 terrorist attacks.

The US military had earlier recommended conspiracy to commit murder and other charges against Mohammed al-Qahtani, who has been held at Guantanamo Bay, Cuba.

But a military official who must confirm the charges, Susan Crawford, a former military appeals court judge, dismissed them without providing a reason.

Crawford approved the charges against alleged September 11 mastermind Khalid Sheikh Mohammed and four other men accused of conspiring with al-Qaeda to carry out the plot.

All five could face the death penalty if convicted under the military commissions set up by President George W Bush in 2006 for trying suspects in the war on terrorism.

The four other defendants are Ramzi bin al-Shibh, the alleged point of contact between the hijackers and al-Qaeda's leadership, Walid bin Attash, believed to have trained some of the hijackers, Ali Abd al-Aziz Ali, Mohammed's nephew and alleged deputy, and Mustafa Ahmed al-Hawsawi.

The five men are held at a secret location at Guantanamo Bay, where the trials will take place. The Pentagon recommended charges against them in February.

The defendants will be tried together on 169 counts that include conspiracy, murder in violation of the law of war, attacking civilians and civilian objects and terrorism. Other charges are causing serious bodily injury, destruction of property and providing material support for terrorism.

Nineteen hijackers commandeered four airliners to strike two towers of the World Trade Center and Pentagon. A fourth plane crashed in Pennsylvania. The attacks resulted in the deaths of 2,973 people.

Al-Qahtani was believed to be the 20th hijacker but was stopped while trying to enter the United States before the attacks. He was later captured in Afghanistan.

The death penalty cases are likely to intensify criticism of the military commissions, which have been challenged in court. Civil rights advocates have raised questions about the fairness of the tribunals.

Each defendant will be entitled to military defence lawyers and could also hire civilian attorneys. Presiding judges will determine what type of evidence can be used, including information obtained through harsh interrogation methods like waterboarding. The CIA has admitted that it used the technique that simulates drowning while questioning Mohammed.

The defendants will have 30 days to enter pleas and a military panel overseeing the trial could begin hearing motions within 90 days.

There is also not a facility at Guantanamo Bay for carrying out executions, and it remained unclear how the Pentagon would proceed to hand out a death sentence. The military has not executed anyone since 1961. (dpa)