The Iranian scientist who American officials say defected to the United States, only to return to Tehran on Thursday, had been an informant for the Central Intelligence Agency inside Iran for several years, providing information about the country’s nuclear program, according to United States officials…

This account by the Americans, some of whom are apparently trying to discredit Mr. Amiri’s tale of having been kidnapped by the C.I.A., provides the latest twist in one of strangest tales of the nuclear era. It also provides the first hint of how the United States acquired intelligence from Iranian scientists, besides its previously reported penetrations of Iranian computer systems…

“His safety depends on him sticking to that fairy tale about pressure and torture,” insisted one of the American officials, who spoke on the condition that he not be identified while discussing a classified operation to attract Iranian scientists. “His challenge is to try to convince the Iranian security forces that he never cooperated with the United States.”