* Prosecutor cast doubts on validity of the charge
* Defense has argued that conspiracy was not a war crime
* Defendants could still face death penalty on other charges
By Jane Sutton
MIAMI, Jan 18 (Reuters) - The Pentagon appointee overseeing the Guantanamo war crimes court refused on Friday to drop conspiracy charges against five accused plotters of the Sept. 11 attacks despite the chief prosecutor's concerns that the charge might not withstand appeals.
The decision announced by the Pentagon means the alleged mastermind of the hijacked plane attacks, Khalid Sheikh Mohammed, and four other captives could be tried on a charge that the prosecutor acknowledged might not have been recognized as a war crime when the attacks occurred in 2001.
In addition to the conspiracy charge, the defendants face murder and other charges that could lead to their execution if they are convicted in the tribunal at the Guantanamo Bay U.S. Naval Base in Cuba.
The chief prosecutor, Brigadier General Mark Martins, asked the Pentagon appointee, Retired Admiral Bruce MacDonald, to dismiss the conspiracy count last week. The prosecutor said doing so would remove uncertainty that could taint or delay the case.
But MacDonald said on Friday that "dismissal at this time would be premature" because an appellate decision on the validity of the conspiracy charge was still pending in a Washington court.
Defense lawyers have argued for years that conspiracy was not recognized as a war crime in 2001, when al Qaeda operatives slammed hijacked passenger jets into the World Trade Center in New York, the Pentagon and a field in Pennsylvania.
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.
CONVICTION STRUCK DOWN
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.
But the Obama administration said last week that it would fight in court to uphold Bahlul's conspiracy conviction and MacDonald said it would be premature to drop the conspiracy charge in the 9/11 case before the appeals court ruled.
Part of MacDonald's role as "convening authority" for the Guantanamo tribunal is to decide what charges are referred for trial.
Martins, the chief prosecutor, declined to comment on Friday. A Pentagon spokesman, Lieutenant Colonel Todd Breasseale, said the disagreement between MacDonald and the prosecutor was "an example of the legal rigor" and debate that is encouraged within the system.
James Connell, a defense lawyer for Mohammed's nephew, defendant Ali Abdul-Aziz Ali, said it showed that MacDonald lacked the neutrality his role requires and demonstrated the unfairness of the tribunal established to try foreign captives outside the regular U.S. courts.
"They have a stake in this outcome and they've decided that they want these charges to go forward even though the chief prosecutor thinks they're not legally viable," Connell said.
Defense lawyers also have asked the judge presiding over the trial at Guantanamo to drop the conspiracy charge and prosecutors did not object. That request was pending.
In addition to the conspiracy charge, the defendants face seven other charges, including terrorism, hijacking aircraft and murdering 2,976 people.
Mohammed and his nephew Ali are Pakistani citizens. The other defendants are Yemeni citizens Walid bin Attash and Ramzi Binalshibh, and Saudi prisoner Mustafa al Hawsawi.
They were captured in 2002 and 2003 and held in secret CIA prisons before being sent to a detention camp at Guantanamo in 2006.