Updated

The United States has made a third appeal to the Iranian government for information about a former FBI agent who has been missing in Iran for more than a month.

The State Department said Monday it had sent a new diplomatic note to Tehran through Swiss intermediaries, asking about the welfare and whereabouts of Robert Levinson, a U.S. citizen who retired from the Federal Bureau of Investigation in 1998, last seen in early March on Iranian island of Kish.

"Over the weekend, we did send another message to the Iranian government asking for an answer as to what they know of his whereabouts," spokesman Sean McCormack told reporters.

Iran has responded to two previous appeals by asking for more information about Levinson, who was on the island — a Persian Gulf resort area and free-trade zone — on private business when his family and current employer reported him missing.

The State Department replied to those requests last month through the Swiss and had expected a response from Iran by now, McCormack said, adding that U.S. officials still had no credible information about what happened to Levinson.

"From all the information we have been able to gather, we believe that he is in Iran, so we have gone back to the Iranian government after what we thought was a sufficient interval for them to conduct an investigation and asked them to give us an answer," he said.

Last week, Iran's Foreign Ministry said Iranian authorities have been searching Levinson but have not provided any details about the probe.

In Florida, meanwhile, Levinson's wife issued a plea Monday for information on his whereabouts.

"I address this message to those in a position to help me locate my husband, Bob Levinson, or to those who may be able to help bring him home to be with his family. Since the day he went missing, I have not heard from my husband. I still don't know where he is or if he's OK," Christine Levinson said in a statement to The Associated Press. "I need to hear from my husband, to hear his voice, to see his face, and to know that he is still alive." She said "there would be no reason" for Iranian authorities to detain him.

Levinson, 59, was believed to be on the island, known for its beaches, sea turtles and relatively liberal atmosphere, researching a project for an independent filmmaker.

Levinson's disappearance was first reported during the crisis over 15 British sailors and marines seized by Iran from the Gulf, but U.S. officials have said repeatedly there is no connection between the cases.

The United States broke diplomatic ties with Iran after Iranian militant students stormed its embassy in Tehran in 1979 and held its occupants hostage for 444 days. U.S. contact with Tehran is handled by the Swiss.

U.S. citizens are not barred from traveling to Iran but must obtain a visa, although no Iranian visa is required to visit Kish island.