(CNN) -- The family of a former FBI agent missing since a 2007 trip to Iran marked the fourth anniversary of his disappearance Wednesday, their hopes buoyed by U.S. authorities' belief that he's still alive.
Bob Levinson was last seen checking out of a hotel on Kish Island, an Iranian resort. He didn't make a scheduled flight to Dubai, where his family said he was working as a private investigator.
Secretary of State Hillary Clinton said last week that U.S. officials have evidence that Levinson "is being held somewhere in southwest Asia" and asked Iran -- which has denied any knowledge of his whereabouts -- to help secure his release.
"We are encouraged at news that he is alive," Levinson's wife, Christine, said in a statement on the anniversary. "Our family is eagerly waiting for the day we will be reunited."
Levinson, now 63, has missed a daughter's wedding and a granddaughter's birth since his disappearance, his wife said.
"Bob's absence weighs on us every day and is magnified during these once-in-a-lifetime family moments," her statement read. "Our family asks for your continued prayers and support. They mean more to us than words can say."
His family says Levinson's private detective work on a cigarette smuggling case took him to Kish Island, a free trade zone off the Iranian coast. They insist he was working as a private citizen, having retired from the FBI in 1998, and the U.S. government has consistently denied Levinson was working for it.
Levinson's suitcase, which had been left behind at his hotel, was returned to his family. Iranian officials, including President Mahmoud Ahmadinejad, have said they have no idea what happened to the Coral Springs, Florida, resident.
The family has offered a $5,000 reward for information leading to his safe return.
The United States and Iran have had no diplomatic relations since 1979, and for most of the last four years, Washington has lacked reliable information about whether Levinson was alive or dead.
The State Department has sent diplomatic notes to Tehran through the Swiss government, which represents U.S. interests in Iran in the absence of an American diplomatic presence in the country. And Clinton personally passed a note to Iranian officials at a U.N. conference on Afghanistan in 2009, asking them for help in finding Levinson.
But in recent months, the United States has received "clear and convincing proof that Bob Levinson is alive," a government official who requested anonymity because of the sensitive nature of the situation told CNN last week.
CNN had reported that Levinson met with Dawud Salahuddin, an American fugitive who lives in Iran, shortly before his disappearance.
Salahuddin -- known in Iran as Hassan Abdulrahman -- converted to Islam and was given refugee status in Iran after admitting in interviews to killing Ali Akbar Tabatabai, a former Iranian diplomat under the shah, in Maryland in 1980, he told CNN in a 2007 interview.
Salahuddin was detained by Iranian officials in plain clothes and taken away from the room he shared with Levinson to be interrogated about his Iranian passport, he said in the interview. When he was freed the next day, he said, he was told by officials that Levinson had returned to Dubai.
Senior U.S. officials have told CNN that they believe Salahuddin met with Levinson, but do not believe him to be a credible source of information on Levinson's whereabouts.
CNN's Susan Candiotti and Ross Levitt contributed to this report.