* Five suspects awaiting trial at Guantanamo naval base
* Ruling cast doubt on whether conspiracy was a war crime
* Defendants could still face death penalty on other charges
The chief prosecutor for the Guantanamo war crimes tribunal recommended on Wednesday that the Pentagon drop a conspiracy charge against five prisoners accused of plotting the Sept. 11 attacks on the United States in 2001.
The prosecutor, Brigadier General Mark Martins, expressed doubts that the conspiracy charge would withstand legal appeal.
If that charge is dropped, the defendants would still face seven other charges in the tribunal at the Guantanamo Bay U.S. Naval Base in Cuba, including charges of murdering 2,976 people in the attacks, carried out by al Qaeda operatives using hijacked planes.
They could still be executed if convicted of planning and executing the attacks that propelled the United States into an ongoing global war against al Qaeda and its affiliates.
The defendants include the accused mastermind of the Sept. 11 attacks, Khalid Sheikh Mohammed, who is alleged to have been al Qaeda's operations chief.
Defense lawyers had long argued that conspiracy was not recognized as a war crime when the attacks occurred in 2001. The defendants are being tried under a law passed by the U.S. Congress in 2006 and revised in 2009, which designated conspiracy and providing material support for terrorism as war crimes.
In October, a U.S. appeals court in Washington struck down the material support conviction of deceased al Qaeda leader Osama bin Laden's driver, former Guantanamo prisoner Salim Hamdan, on grounds that the charge could not be applied retroactively to events that occurred in 2001 and earlier.
A pending appeal on behalf of another Guantanamo convict, al Qaeda videographer Ali Hamza al Bahlul, was expected to bring a similar ruling on the conspiracy charge.
The Obama administration on Wednesday indicated it would fight to uphold Bahlul's conviction on that charge, a decision that could eventually put the case before the U.S. Supreme Court.
Martins said dropping the conspiracy charge from the 9/11 case "would remove an issue that could otherwise generate uncertainty and delay resulting from prolonged litigation in the ongoing capital prosecution."
He made the request to the Pentagon appointee overseeing the Guantanamo prosecutions, retired Vice Admiral Bruce MacDonald.
"There is a clear path forward for legally sustainable charges," Martins said in a news release. "The remaining charges are well-established violations of the law of war and among the gravest forms of crime recognized by all civilized peoples."
The defendants are accused of recruiting, training and funding the hijackers who slammed commercial jetliners into the World Trade Center in New York, the Pentagon and a field in Shanksville, Pennsylvania.
They were captured in 2002 and 2003 and held in secret CIA prisons before being sent to a detention camp at Guantanamo in 2006. Efforts to prosecute them have moved in fits and starts amid controversy over the fairness of the tribunals set up to try non-U.S. citizens outside the regular court system.
The five men are scheduled to appear before a military judge on Jan. 27 for the next pre-trial hearing at Guantanamo.
Mohammed and his nephew, defendant Ali Abdul Aziz Ali, are Pakistani citizens. The other defendants are Yemeni citizens Walid bin Attash and Ramzi Binalshibh, and Saudi captive Mustafa al Hawsawi.
The remaining charges against them are attacking civilians and civilian objects, murder in violation of the law of war, destruction of property in violation of the law of war, hijacking aircraft, intentionally causing serious bodily injury, and terrorism.
Martins said that dropping the conspiracy charge now "would reduce the potential risks in the prosecution of the 9/11 attacks and allow the case to move forward without unnecessary delay."
Only seven cases have been completed in the Guantanamo court and four of them involved only charges of conspiracy and material support.