Parliament's Foreign Affairs committee is studying claims by the legal charity Reprieve that the CIA detained suspected terrorists at a base on Diego Garcia, an island leased from Britain by the United States.
Reprieve claims that at least three so-called "ghost" prisoners -- Khalid Sheikh Mohammed, Abu Zubaydah and Hambali -- were held on the island with the consent of British authorities.
President Bush acknowledged in September 2006 that the CIA had held the three and 11 others in secret prisons outside the United States, but did not say where the detention facilities were located.
Sheikh Mohammed is the suspected mastermind of the Sept. 11 attacks; Zubaydah, an alleged conduit between Osama bin Laden and many al Qaeda cells; and Hambali, a suspected link between bin Laden and regional terrorist group Jemaah Islamiyah.
Reprieve alleges that those and others were subject to torture and cruel, inhuman and degrading treatment. The group also named two amphibious assault ships, the USS Bataan and the USNS Stockham, as being possible sites of "floating prisons" moored offshore from the island.
"There have been repeated, credible and concurrent claims that Diego Garcia has played a major role in the U.S. system of renditions and secret detention," documents submitted to the committee said.
Reprieve subsequently states that the U.K. government failed to adequately investigate earlier accusations, and is potentially complicit in crimes against humanity.
But Britain's Foreign Office said in a statement the U.S. has given repeated assurances "no detainees, prisoners of war or any other persons" have been held on Diego Garcia, or have transited through the island. British officials insist they must be notified by Washington if any civilians are held on the base.
Although Diego Garcia is 60 miles away from any other island, the entire population of the Chagos archipelago -- 2,000 people according to the islanders, 1,000 according to the British government -- was relocated between 1967 and 1973.
In June, a report authored by Dick Marty, a Swiss senator and former prosecutor tasked by the Council of Europe, a human rights watchdog, raised the possibility that the island-territory was used as a processing center for detainees.
Andrew Tyrie, a British lawmaker, has urged the committee to investigate, claiming U.S. assurances are worthless.
"These assurances come from the same government that invented the rendition program, authorized the use of techniques that all in the civilized world would call torture, and continues to hold hundreds in the moral and legal black hole of Guantanamo Bay," he said.
In related documents, Reprieve also asserted that the logs of U.S. rendition flights indicated 23 stopovers in the Turks and Caicos en-route to or from known detention sites, many between Guantanamo Bay and other secret prisons.
Indictments of more than 30 people have been passed down in Germany and Italy relating to the Turks and Caicos flights for their role in renditions.
Associated Press Writer David Stringer contributed to this report.