(Adds sentencing and base background, prosecutor quote)
By Medina Roshan
FORT MEADE, Md., Feb 20 (Reuters) - The brother-in-law of a Sept. 11, 2001, aircraft hijacker pleaded guilty in a U.S. military court on Thursday to plotting with al Qaeda to blow up oil tankers in the Middle East.
The guilty plea by Ahmed al Darbi, a 39-year-old Saudi, marks a victory for U.S. military prosecutors who have battled legal troubles surrounding the tribunal system at the Guantanamo Bay U.S. Naval Base in Cuba.
Judge Mark Allred, an Air Force colonel, accepted al Darbi's plea to all six war-crime charges, made through his attorney Ramzi Kassem.
"This commission finds you guilty of all the charges and specifications," Allred told al Darbi, who had a short beard and wore a white shirt and tie.
The maximum penalty for al Darbi is life in prison and a fine, but under a plea agreement, he will serve another nine to 15 years.
Sentencing will take place in 3-1/2 years, giving al Darbi time to complete the terms of his plea agreement, which include cooperating with U.S. authorities.
Under the terms of his plea deal, al Darbi would be repatriated to Saudi Arabia in four years, or six months after his scheduled sentencing, to serve out his term in a Saudi prison.
Captured in 2002, al Darbi had faced charges that included attempted terrorism, terrorism and attacking civilians.
He was accused of working as a weapons instructor at an al Qaeda camp in Afghanistan in the late 1990s and of meeting al Qaeda leader Osama bin Laden there.
Al Darbi was suspected of using al Qaeda money to buy a boat and GPS navigational devices, and of helping al Qaeda operatives obtain travel documents.
He was charged with abetting a plot to bomb civilian tankers in the Strait of Hormuz and off Yemen from 2000 to 2002. He was accused of assisting in the 2002 plot to bomb a French oil tanker off Yemen, which killed a crewman and dumped tens of thousands of gallons of oil into the Gulf of Aden.
Al Darbi was accused of working with Guantanamo detainee Abd al-Rahim al Nashiri to carry out the attacks.
Nashiri, a Saudi, is charged with masterminding a suicide bombing in 2000 on the USS Cole at Aden, Yemen, that killed 17 U.S. sailors.
The guilty plea and evidence "affirm that (al Darbi's) nearly 12 years of detention as an unprivileged belligerent under the law of war have been grounded in strong legal authority and fact," Brigadier General Mark Martins, the chief prosecutor, said in a statement.
Al Darbi is married to a sister of Khalid al Mihdar, who helped hijack American Airlines Flight 77, which crashed into the U.S. Defense Department's Pentagon headquarters in 2001.
The hearing at Guantanamo Bay was transmitted via closed-circuit television to Fort Meade, an Army base outside Washington.
Al Darbi's guilty plea is the sixth by a Guantanamo defendant. Ali Hamza al Bahlul, the former publicist for slain al Qaeda leader Osama bin Laden, and Salim Hamdan, bin Laden's driver, have had their convictions overturned by an appeals court.
The Guantanamo Bay prison was opened by President George W. Bush in 2002 to hold terrorism suspects rounded up overseas. It has 155 prisoners, most of whom have languished for a decade or more without being charged or tried.
President Barack Obama has pressed forward with military tribunals in the hope that they will add legal validity to the detentions while he tries to persuade Congress to close the prison.
The military commissions have been beset with procedural delays and questions about the system's legitimacy.
Among the cases that have been hindered are efforts to prosecute the group of suspected plotters of the four coordinated attacks on the United States on Sept. 11, 2001, in which nearly 3,000 people died. (Writing by Ian Simpson; Editing by Bernadette Baum and Jonathan Oatis)