Four years after a retired FBI agent mysteriously vanished inside Iran, U.S. officials have received proof he is alive, a remarkable development that has dramatically intensified secret negotiations 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 rare hopeful sign in a case that had seemingly gone cold.

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 63-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 since shortly after it arrived but delayed reporting it because officials said any publicity would jeopardize the ability to get Levinson home. The government announced Thursday afternoon that there were signs he was alive, so the AP published its story.

The AP is not disclosing the nature of the proof because officials believe that would hurt efforts to free him.

The current and former officials who discussed the matter and the pending announcement insisted on anonymity because the issue is so sensitive.