Two human rights groups called Saturday for Britain to launch a new inquiry into claims the United States used the remote British outpost Diego Garcia to detain and interrogate terrorism suspects. Campaigners from Britain's Liberty and the American Civil Liberties Union said in a letter to Britain's Foreign Secretary David Miliband that they suspect the Indian Ocean island has been used to hold prisoners – despite repeated denials from Washington and London. Britain's Foreign Office has said that the US has offered assurances that the outpost – halfway between Africa and Southeast Asia – has not been used to detain suspects. But in February the US acknowledged previous denials that the island had never been used by so-called extraordinary rendition flights had been wrong. The State Department said it had misled the British government and confirmed two suspects had been on flights that stopped to refuel on Diego Garcia en route to Guantanamo Bay and Morocco in 2002. Britain demanded checks on a list of 391 other flights highlighted by human rights groups and lawmakers, but the US said it found no evidence of other cases where Diego Garcia had been used. Campaigners insist the island has been used to hold suspects, citing claims by former US counterterrorism adviser Richard Clarke that the base was likely used for interrogations. “It seems that previous failures by the US to inform the UK government about the use of Diego Garcia for rendition flights might only be part of the picture,” Liberty and the ACLU said in the joint letter. The letter, signed by Liberty director Shami Chakrabarti and ACLU executive director Anthony Romero, calls on Miliband to order a full public inquiry into the use of the island. “We will not rest without real answers to the scandal of torture flights and inhuman interrogation in freedom's name,” Chakrabarti said. Britain has leased the island to the US to be used as a military base since the 1970s. Entry to the island is barred except by permit. Lawmakers pledged last month to make a new study of the movements of planes and ships traveling to the island, amid lingering concerns over its use.