* First word of al Qaeda suspect's whereabouts since 2005
* Wanted by Spain over Madrid bombings
* U.S. role unclear, officials decline comment
LONDON, June 10 (Reuters) - Lawyers for a senior al Qaeda ideologue wanted in Spain for possible links to 2004 bombings say he is in detention in Syria, in the first firm indication of his whereabouts following his capture in Pakistan in 2005.
Human rights attorney Clive Stafford-Smith told Reuters that Mustafa Setmariam Nasar's presence in Syria had come to light only recently but he may have been held there for some years.
Stafford-Smith said Nasar, also known as Abu Musab al-Suri, appeared to have been under effective U.S. control in a secret system of detention and transfers before surfacing in Syria, which is listed by Washington as a state sponsor of terrorism.
Washington has not confirmed Nasar has been in U.S. custody. U.S. officials contacted on Wednesday declined to comment.
There was no immediate comment from officials in Damascus.
Nasar, 50, who holds a Spanish passport through marriage, is suspected of involvement in the March 2004 train bombings in Madrid that killed 191 people. He is wanted by Interpol, which at Spain's request issued a global 'red notice' seeking his arrest.
He is best known to Islamist insurgent networks as the author of a 1,600-page treatise, "Call to Global Islamic Resistance". Published online in January 2005, it has inspired many radical groups including al Qaeda, according to experts.
In brief remarks to Reuters, Nasar's wife, Elena Moreno, said she had also come to believe her husband was probably in Syria, following what she called recent but unofficial confirmation.
"Most probably - I don't know when and I don't know how -- it seems that he may have been taken to Syria," she said.
She said she had never received any official word from any government about his whereabouts following his capture.
However, over the years the family had had unofficial indications of his presence at several locations, one of which was the Indian Ocean island of Diego Garcia, she said.
While the island is a British territory, it is dominated by a U.S. military base established during the Cold War, strategically placed within reach of the Gulf area and South Asia. Long-range U.S. military planes are deployed there.
U.S. ally Britain had long said it was not aware of its territory being used for 'renditions', in which suspects in the fight against al Qaeda have been captured in one country and secretly transferred to another for interrogation.
But British Foreign Secretary David Miliband apologised to parliament last year after it emerged that two U.S. planes carrying suspects on rendition flights had landed and refuelled at the U.S. base on Diego Garcia in 2002 despite previous British government denials based on U.S. assurances.
Nasar's arrest was never officially confirmed by the Pakistani government, but Pakistani intelligence officials told Reuters in 2006 that he was caught in the southwest city of Quetta on Oct. 31, 2005, after a gunbattle.
A senior intelligence source said at the time that he was handed over to the Americans in March 2006 after repeated demands by the United States.
Stafford-Smith said Nasar deserved the opportunity to clear his name in a trial, adding that the Syrian-born Islamist was a theoretician rather than a practitioner of violence. (Additional reporting by Tabassum Zakaria in Washington) (Editing by Mark Trevelyan)
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