A Saudi detainee at the US prison in Guantanamo Bay has pleaded guilty to plotting with al-Qaeda to blow up a French oil tanker in Yemen in 2002.
Ahmed Mohammed Ahmed Haza al-Darbi, the brother-in-law of one of the September 11, 2001 plane hijackers, admitted to planning, aiding and supporting an attack on the MV Limburg which killed a Bulgarian sailor, injured a dozen and caused a large oil spill in the Gulf of Aden.
Darbi, who has been held in Guantanamo for more than a decade, likely faces up to 15 more years in prison, the chief prosecutor, Army Brigadier General Mark Martins, said in a statement.
Some of that time could be served in his native Saudi Arabia, the AFP news agency reported.
His lawyer Ramzi Kassem announced Darbi was pleading guilty to charges of terrorism and to attacking civilians and civilian targets before the US military judge at Guantanamo.
"This moment is bittersweet," said Kassem, whose client agreed to cooperate with prosecutors as part of the plea deal.
They accuse Darbi of having met with and worked for fellow Saudi Guantanamo detainee Abd al-Rahim al-Nashiri, who faces the death penalty on charges he masterminded the MV Limburg attack and the 2000 attack against the USS Cole in Yemen that left 17 dead.
By pleading guilty, Darbi, 39, could become a key witness against Nashiri, whose trial is likely to open in September.
Darbi "has pledged to be law-abiding and to cooperate fully and truthfully with authorities," Martins said, adding that soon after his arrest, Darbi began "divulging some useful information to authorities about his involvement in terrorist activities".
Speaking a mix of Arabic and English, Darbi admitted supplying visas, boats and other necessary equipment to those who carried out the attack on the MV Limburg.
Under the terms of the plea deal, Darbi will not be officially sentenced for another 3.5 years, judge Mark Allred said at the hearing, which was retransmitted for reporters at the US military base in Fort Meade, Maryland, outside Washington.
The delay, which has been standard for Guantanamo detainees who plead guilty, means he would still be at the US prison in Cuba when Nashiri's trial begins.
Once Darbi is formally sentenced, he could be released from the Guantanamo jail to serve the remainder of his sentence in Saudi Arabia, Martins said.
A Pentagon spokesman explained he would serve a minimum of five more years in a Saudi jail, depending on whether his behaviour from this point forward was deemed cooperative, AFP reported.
At the hearing, Darbi emphasized that by the time the MV Limburg attack was carried out, "I was already detained at that time, for four months."
The oil tanker was bombed on October 6, 2002 and Darbi was captured in June of that year, according to military documents unveiled by WikiLeaks.
Nevertheless, Darbi has admitted to being "legally responsible" for all the charges, which could have seen him jailed for life without the plea deal.
He waived his right to appeal his conviction as well as to contest his capture and decade-plus detention in Guantanamo, and said: "I agree to cooperate fully and truthfully with the government.
"This cooperation includes, but is not limited to, providing complete and accurate information in interviews, depositions, and testimony wherever and whenever requested by prosecutors."