US guard at military-run Camp Delta prison in Guantanamo Bay  ( file photo)

The United States has dropped charges against the alleged "20th hijacker" in the September 11th, 2001, terror attacks on the United States.

The Defense Department says charges against Mohammed al-Qahtani of Saudi Arabia were dropped without prejudice, meaning they can be filed again later.

Charges against five other suspects in the attacks were referred to trial. Those suspects include the alleged mastermind of the attacks, Khalid Sheikh Mohammed.

Pentagon spokesman Bryan Whitman could not say why the charges against al-Qahtani were dropped, but did say that the reasons could include the nature of the charges, the evidence and commission rules, among other potential factors.

Whitman says the five suspects referred to trial should be arraigned within 30 days, and the trial could begin within 120 days.

The Pentagon says the five men will be tried jointly in Guantanamo Bay, Cuba, and that they face the possibility of being sentenced to death.

U.S. prosecutors said al-Qahtani did not take part in the attacks because he was denied entry into the United States by an immigration agent. Al-Qahtani recanted a confession he made at Guantanamo Bay, Cuba, alleging it was made after he was tortured and humiliated.

Prosecutors filed murder and war crimes charges in February against the suspects, who also include Walid bin Attash, Ramzi bin al-Shibh, Ali Abd al-Aziz Ali and Mustafa Ahmed al-Hawsawi.

All of the men are being held at the U.S. military facility in Guantanamo Bay.

The September 11th, 2001 attacks killed nearly 3,000 people after hijacked jetliners crashed into the World Trade Center in New York, the Pentagon outside Washington, and a field in the eastern state of Pennsylvania.

Some information for this report was provided by AFP and AP.