WASHINGTON (CNN) -- Iran may be holding a former FBI agent in a bid to exchange him for Iranians seized by U.S. troops in Iraq in 2007, a U.S. senator suggested Tuesday.

Christine Levinson, wife of Robert Levinson, went to Tehran in 2007 to try to learn her husband's fate.

Robert Levinson disappeared from Iran's Kish Island in March 2007. Iranian authorities have repeatedly said they don't have any information on him, but that is widely doubted in the United States.

"On several diplomatic occasions when Bob Levinson's name has been brought up to Iranian officials, the standard answer is, 'We don't know anything about that.' But the next thing out of the Iranian officials' mouths are to discuss the matter of the Iranians held by the Americans in Irbil, Iraq," Sen. Bill Nelson, D-Florida, told reporters. "You can draw your own conclusions."

U.S. troops arrested five Iranians accused of being members of an elite Iranian military unit during a January 2007 raid in the Kurdish city of Irbil. They were accused of supporting Shiite militias in Iraq, but Iran said they were diplomats and accused the United States of violating international law by raiding a consulate.

The United States said the men were taken at a liaison office that lacks diplomatic status. Two were released the next November.

Nelson and Florida Democratic Rep. Robert Wexler, whose district includes Levinson's Coral Springs home, are trying to jump-start efforts to find the missing man. Wexler said Iranian authorities have not assisted the Levinsons, saying they have "zero knowledge" of his whereabouts and are stonewalling "any effort to gain pertinent information."

Levinson's wife, Christine, said her husband was working as a private investigator on a cigarette smuggling case when he disappeared.

"It's been extremely difficult for my family," she said. "We hope to resolve this as soon as possible."

Wexler and Nelson said they plan to introduce legislation in their respective houses calling on Iran to cooperate with the United States and come up with information about Levinson. Nelson said Iran could use Levinson's case as a show of good will toward the incoming Obama administration, which has said it wants to engage Iran after three decades without diplomatic relations.

"We hope that Iran will take this moment in time to show a change in the relationship with the United States by doing this humanitarian gesture for this family," he said.

In a January 22 letter to Secretary of State Hillary Clinton, Wexler asked that the case be raised "at the highest levels." Nelson raised Levinson's status during Clinton's confirmation hearing.

Nelson said the Iranian ambassador to the United Nations has discussed Levinson's case with him under the "fiction" that he represents the people of Florida, not the U.S. government. But he said the Iranians have never acknowledged holding the former federal agent.

Levinson had been working as a private investigator in Dubai and was last heard from on March 8, 2007. His family said he checked into a hotel on Kish Island and then checked out the next day to go back to the United States, but he never boarded his flight.

Levinson family members have met with local Iranian officials and have traveled to Iran to retrace his steps. Christine Levinson also flew to the United Nations in September to ask questions about her husband, whom she says suffers from diabetes and high blood pressure. The family has offered a $5,000 reward for information leading to his safe return.

The State Department and FBI have consistently denied Levinson was working for the government and have demanded Iran free Levinson -- if it is holding him. State Department spokesman Robert Wood said the U.S. government has called on Iran to provide more information about the matter "for some time."

"His family's extremely concerned, as you can understand, that no information has been forthcoming," Wood said. "So, again, we call on the Iranians to provide whatever information they have on Mr. Levinson so that we can hopefully return this gentleman to his family."
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question: Who has been missing nearly two years?, answer: Robert Levinson | question: Who disapeared almost two years ago?, answer: Robert Levinson | question: Who seeks new attention from Obama administration?, answer: Christine Levinson, | question: What does the Florida congressman and senator seek?, answer: bid to exchange him for Iranians | question: Who disappeared almost two years ago?, answer: Robert Levinson | question: Where did Levinson vanish from?, answer: Iran's Kish Island | question: How long ago  did Levinson disappear?, answer: March 2007. | question: Where did Levinson disappear from?, answer: Kish Island | question: What did the Iranians say?, answer: 'We don't know anything about that.' | question: How much knowledge do Iranians say they have about Levinson's whereabouts?, answer: "zero knowledge" | question: What senator is seeking attention from the White House?, answer: Sen. Bill Nelson, | question: What did the Iranians say about missing man?, answer: "zero knowledge"