NEW YORK- Charging that the U.S. government was complicit in the forced disappearance of an influential Muslim scholar four years ago, human rights groups in the U.S., the U.K., and Switzerland have asked the U.N. to investigate.

In a letter to the U.N., the organizations say Mustafa Setmariam Nassar, a Spanish citizen, was arrested by Pakistani officials and handed over to U.S. officials in Oct. 2005 and has not been heard from since.

The letter was sent to the U.N. Special Rapporteur on Torture, Manfred Nowak, the U.N. Special Rapporteur on the Promotion of Human Rights While Countering Terrorism, Martin Scheinin, and the U.N. Working Group on Enforced or Involuntary Disappearances. It was signed by the American Civil Liberties Union (ACLU), the London-based legal charity Reprieve, and Alkarama in Geneva.

In Jun. 2009, in response to an ACLU request for information about Nassar's whereabouts, the U.S. Central Intelligence Agency (CIA) said it could "neither confirm nor deny the existence or nonexistence of records" concerning Nassar.

Steven M. Watt, staff attorney with the ACLU Human Rights Program, told IPS, "Mr. Nassar's wife and children just want to know if he is still alive and where he is." He said that "Requests for information about his forced disappearance, nearly four years ago, have been ignored by the U.S. government, and his family now has no other choice but to turn to the international community for assistance in their quest."

He added, "The CIA should be held accountable. It should allow his family to know what happened to him and where he is. Or deny that it had any involvement in his disappearance."

The letter asks the U.N. to raise Nassar's case with the U.S. government and other governments that may have assisted the U.S. in Nassar's disappearance, or may have information that could assist in locating him.

The organizations acknowledge that information about Nassar's disappearance is scarce. But they say "the known details suggest he was a victim of the unlawful extraordinary rendition" program, which enabled the U.S. - with the assistance of other governments - to kidnap and transport foreign nationals suspected of terrorism to secret overseas detention facilities for interrogation and torture.

Official U.S. documents and media reports indicate that the U.S. had long been interested in capturing Nassar - suspecting him of involvement in certain terrorist acts, but never charging him with a crime. In Jan. 2005, months before his reported capture in Pakistan, the U.S. Embassy in Pakistan announced a 5 million dollar reward for information leading to Nassar's capture, which was withdrawn around the time of his reported capture.

The U.S. National Counterterrorism Centre confirms Nassar's capture in Nov. 2005, and media reports indicate that Nassar was later held for a time at a U.S. military base on the British-owned island of Diego Garcia in the Indian Ocean.

The Reprieve group also demanded the British government reveal details of the secret illegal detention of what it called the 'ghost' prisoner on Diego Garcia.

Reprieve says Nassar was then sent to Syria, where he was "held incommunicado in shocking conditions and almost certainly tortured."

The group added, "The U.K. shares responsibility for Nasser's disappearance because of its complicity in his 'ghost' detention on the Diego Garcia and elsewhere."

It has written to the U.K. government on behalf of Nasser's wife to "demand the U.K. fulfils its legal obligation to investigate his disappearance."

Reprieve's Director, Clive Stafford Smith, said: "Enforced disappearance is a crime most associated with ruthless South American dictatorships, yet here we have the U.S. and British governments embroiled in the same dirty deeds. Kidnapping is a crime in anyone's language, and it is about time that powerful governments are held to account for their crime against Mustafa Nasser."

Diego Garcia has featured prominently in at least two other current cases. In one, Reprieve is suing the U.K. government on behalf of British resident Binyam Mohamed, a recently released Guantanamo detainee, for allowing the island's airbase to be used to facilitate Mohammed's "rendition," by landing to refuel.

Mohammed was first rendered from Pakistan to prison in Morocco, and finally to Guantanamo. The group claims he was tortured in all three locations.

David Miliband, the British Foreign Secretary, has argued before the U.K. High Court that it must suppress evidence of torture because the U.S. has threatened to discontinue sharing intelligence with the British if it discloses such evidence. The specific evidence in this case is a seven-paragraph document that Reprieve says has no intelligence or national security value but includes American admissions that they tortured Mohammed.

The High Court Justices said that such a threat was not based in law. "I mean, it is an exercise of naked political power," Lord Justice Thomas said, adding, "That is not constitutional, it is the use of naked political power." Under British law, it is a criminal offence to suppress evidence of torture.

In the second case, Mohamed and four other now-released Guantanamo detainees are suing a Boeing Company subsidiary, Jeppesen Dataplan, for knowingly assisting in Mohammed's rendition by providing the CIA with logistical support for the flight that landed on Diego Garcia for refuelling.

In the Nassar case, responding to a Jun. 2009 request from a Spanish judge for information on Nassar's whereabouts, the U.S. Federal Bureau of Investigation (FBI) said it was not holding him in the U.S. but did not address whether he was being held in U.S. custody elsewhere. Asserting that the information is classified, the U.S. government has also refused to answer direct requests for information about Nassar's whereabouts made by his wife, Spanish citizen Helena Moreno Cruz.

"I have been bringing up four children without their father for nearly four years now," she said. "They keep asking about dad and I have no idea what to tell them anymore - I don't even know if their father is still alive."

"If my husband is suspected of doing anything wrong, he should get his day in court. If he isn't, he should be let go. No one deserves to be treated like this," she added.

Nassar, a 42 year-old Spanish citizen of Syrian origin, is considered an influential Islamic theorist and intellectual. He has written a number of books and articles on Islam and jihad.

Law enforcement authorities in the U.K., Spain, and the U.S. have long suspected Nassar of having been involved in a number of terrorist acts - including the Sep. 11, 2001 attacks against the U.S. - though he has never been charged with a crime.

In the early 1980s, Nassar fled Syria following his involvement in a failed attempt by the Muslim Brotherhood to overthrow the government then in power there.

The letter to the U.N. says the former U.S. administration of George W. Bush pursued Nassar at least since Nov. 2004, when it offered a 5 million dollar reward for information relating to his capture as part of its "Rewards for Justice" program.