Three days after The Associated Press reported that former FBI agent Robert Levinson was on quasi-official contract with the CIA when he disappeared from the Iranian island of Kish, U.S. Secretary of State John Kerry said the U.S. government hasn't given up its six-year search for him. On Sunday, Kerry told ABC's Martha Raddatz that he has "personally raised the issue not only at the highest level that I have been involved with but also through other intermediaries."

At this point, Kerry said, the U.S. is seeking "proof of life" for the 65-year-old former FBI and Drug Enforcement Administration employee. "To suggest that we've abandoned him, or anybody has abandoned him, is simply incorrect and not helpful."

U.S. officials have privately believed for a long time that Levinson was nabbed by Iranian security before a scheduled meeting with a U.S. fugitive, Dawud Salahuddin, about money laundering allegations against Iranian officials. The New York Times — which, like the AP, sat for years on emails outlining Levinson's CIA ties, so as not to jeopardize his safety or sabotage efforts to free him — said in a long profile Saturday that U.S. officials have privately long believed that Levinson has been held by "a group tied to Iranian religious leaders, possibly the Revolutionary Guards."

Iran has flatly denied holding Levinson, and Iranian Foreign Minister Mohammad Javad Zarif unequivocally repeated the denial on Sunday. "We know that he is not incarcerated in Iran," he told CBS's Face the Nation. "If he is, he is not incarcerated by the government. And I believe the government runs, pretty much, good control of the country."

Salahuddin — an American who fled to Iran in 1980 after killing Ali Akbar Tabatabai, a former aide to the shah, outside Washington — also denies any involvement in Levinson's disappearance, for whatever that's worth. "Personally, I have nothing to say except that I did not contact any Iranian official and would rather be dead than an asset for the world's leading terror apparatus, i.e., the CIA," Salahuddin told NBC News.

NBC News producer Ira Silverman had introduced Levinson to Salahuddin, The New York Times reports, and set up their meeting, suggesting to Levinson that the fugitive assassin might inform on Iran for the U.S.

In 2010, somebody emailed Levinson's family a video of Levinson pleading for help from the U.S. government and saying he was running low on diabetes medicine. The video was sent from an internet cafe in Pakistan, and Pashtun wedding music was playing in the background. In March 2011, then–Secretary of State Hillary Clinton issued a statement saying the U.S. had evidence Levinson was "somewhere in southwest Asia."

Now officials say that was a ruse to allow Iran an opening to release Levinson without being blamed for his capture. A month later, somebody in Afghanistan emailed Levinson's family photos of him in an orange jumpsuit and chains — similar to prison garb at America's prison camp at Guantanamo Bay — with a series of messages. "Then, a surprising thing happened," says the AP's Matt Apuzzo: "Nothing."

Faced with a cold case, analysts from the CIA, FBI, and State Department came up with new theories, including that Levinson was abducted by the Russian criminals he spent much of his career tracking down. Maybe he really was in the mountainous border area shared by Iran, Pakistan, and Afghanistan. But all roads seemed to lead back to Iran.

Unfortunately, there is one theory that fits the known facts and denials: Levinson is dead. He was an overweight diabetic with high blood pressure when he disappeared, and he looked pretty ill in the final photos sent to his family. Iran reportedly executes prisoners fairly often, but it's also possible he died during a harsh interrogation session. If he is dead, Iran is also technically not lying: They are not keeping him incarcerated. U.S. officials acknowledge "there is no upside for the Iranians to admit he died in their custody," says Adam Goldman at The Washington Post.

It's an open question whether top U.S. officials knew about Levinson's contract with a group of rogue CIA analysts — after an internal investigation, 10 employees were disciplined and three fired — or when they knew it. But in spy novels and movies, the line is always the same for undercover agents: If you are caught, your government will disavow any knowledge that you even exist. Unlike in espionage fiction, though, the U.S. by all accounts has made a huge effort to find Levinson and bring him back alive. Also unlike in most films and books, America has failed.