A retired FBI agent who vanished in 2007 while investigating a smuggling case in Iran is apparently still alive, Secretary of State Hillary Rodham Clinton said Thursday in a statement that deepened the mystery surrounding the man's nearly four-year disappearance.

Robert Levinson, 63, who disappeared during a visit to the Iranian resort island of Kish, apparently is being held "somewhere in southwest Asia," Clinton said, citing unspecified evidence received by the State Department.

Clinton did not elaborate on the nature of the evidence or the circumstances of his detention but called on the Iranian government to assist in his repatriation.

"As the government of Iran has previously offered its assistance in this matter, we respectfully request the Iranian government to undertake humanitarian efforts to safely return and reunite Bob with his family," Clinton said.

The evidence, if borne out, would represent the first news about Levinson's well-being in years. Levinson, who worked as a private detective after he retired from the FBI in 1998, was said by family members to have been in Iran to investigate a cigarette-smuggling case on behalf on several corporations. His disappearance March 9, 2007, has long been the subject of speculation and rumors, with Iranian defectors and others suggesting that he was imprisoned on suspicion of spying or perhaps kidnapped or killed by mobsters.

The wording of Clinton's statement prompted speculation that Levinson may have been transferred to another country in the Middle East where Iran has proxy groups or influence.

Dawud Salahuddin, an American fugitive living in Iran, said in 2007 that he met with Levinson before his disappearance and that the retired agent was seized by Iranian security forces on Kish, a duty-free resort island off Iran's southern coast. A few weeks afterward, Iran's state-sponsored Press TV also reported that Levinson was "in the hands of Iranian security forces."

But until now there has been no official word from the Iranian government about his fate. Iran refused a request by the United States later that year to allow Swiss diplomats to try to find his luggage and look for clues to what happened to him.

U.S. officials have repeatedly pressed Iran for information about Levinson, a Florida resident and 27-year veteran of the FBI, and members of his family have traveled to Tehran to appeal to the Iranian government for help. At her confirmation hearing in 2009, Clinton was urged by Sen. Bill Nelson (D-Fla.) to take up the case.

"We think he is being held by the government of Iran in a secret prison," Nelson said.

Staff researcher Julie Tate and SpyTalk columnist Jeff Stein contributed to this report.