British human rights campaigners Reprieve have urged the Spanish authorities to board and search US torture ship USS Bataan after it moored at the Palma de Mallorca holiday resort.
Reprieve said on Friday that the USS Bataan is one of the US government's most infamous "floating prisons" and will remain at the island until Saturday.
At least nine prisoners including John Walker Lindh, David Hicks and Ibn Al-Sheikh Al-Libi, who recently died in mysterious circumstances in Libyan custody, are confirmed to have been held aboard the USS Bataan.
Reprieve pointed out that, in January 2002, Mr Al-Libi was flown to the ship, which was then cruising the northern Arabian Sea, before his interrogation began.
From there, he was rendered to Egypt where he was forced under torture to confess that al-Qaida and Saddam Hussein were in league on weapons of mass destruction.
Details regarding the operation of prison ships have emerged through a number of sources, including the US military and other administration officials, the Council of Europe, various parliamentary bodies and journalists, as well as the testimonies of prisoners themselves.
Reprieve investigations also suggest that a further 15 ships have been used to hold prisoners beyond the rule of law since 2001. Prisoners are interrogated aboard the vessels and then rendered to other, often undisclosed, locations.
A former prisoner told Reprieve: "One of my fellow prisoners in Guantanamo was at sea on an American ship before coming to Guantanamo. He was in the cage next to me. He told me that there were about 50 other prisoners on the ship.
"They were all closed off in the bottom of the ship. The prisoner commented to me that it was like something you see on TV. The people held on the ship were beaten even more severely than in Guantanamo."
Reprieve investigator Clara Gutteridge said: "Ships have been used by the US to hold terror suspects illegally since the days of president Clinton, so it would be no surprise if this practice continues under Obama.
"The US and Spanish governments, as well as the EU, must urgently reveal what this ship is doing on European territory."