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An Iranian nuclear scientist who claimed he slipped away from his CIA captors has not been seen publicly since his heroic return to Tehran last year and could be facing an investigation as a possible turncoat, according to an opposition website.

The report on the website Iranbriefing.net says that Shahram Amiri is now being held in a Tehran prison, where he allegedly has faced beatings so severe that he had to be hospitalized for a week.

The website's account could not be verified, but it purports to offer the first details on Amiri's fate since he surfaced in the United States six months ago amid an array of contradictory stories between Tehran and Washington.

Amiri's return to Tehran in July was portrayed by Iranian authorities as a propaganda coup.

The nuclear scientist claimed he was kidnapped by American agents in May 2009 while on a pilgrimage in Saudi Arabia. Iran's state media said Amiri had snubbed a fortune in payoffs by his purported CIA captors and then escaped with inside information about America's covert operations against Tehran. Officials said a TV movie was in the works.

U.S. officials, however, say Amiri willingly cooperated and had been paid a total of $5 million by the CIA for "significant" information about Iran's nuclear program.

Amiri turned up in Washington at the Iranian Interest Section at the Pakistan Embassy after a series of baffling videos that presented contradictory stories: one claiming he was on the run from the CIA and the other saying he was studying for a doctorate in the U.S.

Amiri then decided to leave the United States - but without the money - in July, American officials said. His return to Tehran, however, was the last time Amiri was seen in public.

Iranian authorities have not offered any information on his whereabouts and his family and colleagues have made no public statements as to his fate.

The Farsi-language account posted on the Iranbriefing.net website claims to pick up Amiri's trail after his highly publicized return. It cited anonymous family members as saying Amiri was first held in a safe house in Tehran and allowed weekend visits with relatives at the Talaiie cultural center, which is operated by Iran's powerful Revolutionary Guard.

The website quoted family members as being told that Amiri's movements were restricted for "his own safety."

The report says - without citing any source - that Amiri was later moved to a former military lockup, the Heshmatieh Prison, in Tehran, where he allegedly faced harsh interrogations and beatings that left him in a military-run hospital afterward for a week.

The website is operated by the IranBriefing Foundation, which describes itself as a "non-profit human rights organization" based in the United States that focuses on the role of the Revolutionary Guard and other Iranian security agencies. Officials at the group could not be reached immediately for comment.

The reports - if true - would fit into Iran's increasing attention to trying to plug security holes inside its nuclear program, which the West and others fear could lead to the development of atomic weapons. Iran claims it only seeks reactors for power and research.

In October, one of Iran's vice presidents, Ali Akbar Salehi - who is now acting foreign minister - acknowledged that some personnel at nuclear facilities had passed secrets to the West in exchange for payment. Salehi claimed that it "awakened" security forces to impose tighter controls.

Iran also acknowledged that a sophisticated computer worm, known as Stuxnet, had infiltrated systems at nuclear sites, including briefly halting activity at Iran's uranium enrichment lab. Iran's intelligence chief has accused the CIA and Israeli and British spy agencies of being behind the cyber attacks.