WASHINGTON — In his day, Robert A. Levinson was the consummate g-man. As a special agent in Miami, Levinson was was credited in the early 1990s with helping bring down Columbia’s Medellin drug cartel. When he retired from the Bureau after 22 years, he kept working as a private detective. And exactly five years ago, while on the job in Iran, Levinson disappeared.

Now, the FBI is offering a $1 million reward for any information leading directly to Levinson’s safe return. Such a large reward is rare, but FBI officials believe Levinson may still be alive and are hoping the reward will entice someone to come forward.

Levinson traveled to the resort island of Kish in Iran on March 8, 2007, to investigate cigarette smuggling on behalf of a client. He disappeared the next day after checking out of his hotel to fly home. Levinson has not been seen or publicly heard from since. His 64th birthday is Saturday.

In November 2010, three and a half years after his disappearance, Levinson’s family received an emotional 57-second video by e-mail showing a gaunt and bedraggled Levinson in captivity, pleading for U.S. intervention to bring him home. Levinson, who has diabetes and hypertension, said in the video that he was running “very quickly out of diabetes medicine.”

“I need the help of the United States government to answer the requests of the group that has held me,” said Levinson in the “proof-of-life” video. Last December, his family released the video, the first images of Levinson to be made public since his disappearance.

Levinson met briefly on Kish Island with Dawud Salahuddin, an American fugitive who fled to Iran after allegedly assassinating a former aide to deposed Shah Mohammad Reza Pahlavi in Washington in 1980. Salahuddin is thought to be the last person who saw Levinson before his disappearance.

In March 2011, Secretary of State Hillary Rodham Clinton said that the U.S. had received indications that Levinson was being held hostage by an unidentified group in the area that includes the border regions of Afghanistan, Iran and Pakistan.

The Obama administration has repeatedly appealed to Iran for help, but Iranian officials have denied having any role in the former agent’s disappearance or any knowledge about where he is being held.

“There are no words to describe the nightmare my family and I have been living every day,” said Christine Levinson, the missing agent’s wife of 37 years. The Levinsons have seven children and two grandchildren.

This week, a group of retired and current FBI agents gathered outside of the FBI’s Washington Field Office to announce the reward.

“We in the FBI share in your heartache,” FBI Director Robert S. Mueller III told Levinson’s wife. “But we also share in your sense of hope, hope of Bob’s safety, hope of Bob’s health and hope of his homecoming.”

The FBI and other federal agencies this week launched a publicity campaign in southwest Asia to announce the reward. They are using billboards, radio messages and flyers to get the word out, and have set up a confidential telephone tip line in the region.

Levinson worked 28 years for the Justice Department, including six years in the Drug Enforcement Administration. He spent his FBI career in Los Angeles, Miami and New York, where he was known for investigating Italian and Russian mobsters.

Guy Lewis, the former U.S. attorney in Florida, described Levinson as “really smart, enthusiastic, extremely professional and aggressive.”

“This was a guy who could tell you the members of the Medellin cartel before people even knew there was a Medellin cartel,” said Lewis, who worked with Levinson.

James W. McJunkin, the assistant director in charge of the FBI’s Washington Field Office, said there has been no information on Levinson’s well-being and no demands for a ransom. “We hope that this reward will encourage anyone with information about Bob or his captors, no matter how insignificant it seems, to contact the FBI,” McJunkin said.

The FBI posted on its Web site two photographs and a composite of what they think Levinson may look like now and is asking anyone with information to contact the Bureau.

The Levinson family also established a Web site as a possible source of tips and communication: HelpBobLevinson.com. Christine Levinson said that she, her son and her sister traveled to Iran in 2007 to retrace her husband’s steps. She said the family reads every e-mail it receives, hoping to find a clue.

“We hope someday to post the following message: Thank you everyone. Bob is home,” she said.