Washington ? Four years after a retired FBI agent mysteriously vanished inside Iran, U.S. officials have received irrefutable proof he is alive, a dramatic development that has sharply intensified diplomatic efforts to bring him home, The Associated Press has learned.

The U.S. had lacked reliable information about whether Robert Levinson was alive or dead since he disappeared in March 2007 from the Iranian island of Kish. It remains unclear who exactly is holding Levinson or where he is, but the proof that he is alive is a hopeful sign in a case that had seemingly gone cold.

The State Department issued a three-sentence statement by Secretary of State Hillary Rodham Clinton Thursday saying there were indications Levinson was in southwest Asia and asking Iran for help. The AP has learned fuller details after a lengthy investigation into Levinson’s disappearance and the effort to get him back to the U.S.

Iran has repeatedly said it has no information about Levinson, but U.S. diplomats and investigators have long said they believed he was taken by Iranian government agents.

As years passed, many in the U.S. government believed the 62-year-old with diabetes and high blood pressure might have died. But late last year, Levinson’s family received proof that he was alive. Investigators confirmed its authenticity and that it was recent, current and former officials said. Officials say they believe he is still alive.

The AP has known about the proof of life since shortly after it arrived but delayed reporting it because officials said any publicity would jeopardize the ability to get Levinson home.