A U.S. senator introduced a resolution Thursday calling on Iran to allow the FBI to dig deeper into the 2007 disappearance of former agent Robert Levinson.

Sen. Bill Nelson's resolution asks Iranian investigators to confer with the FBI and for the Iranian government to provide the assistance it has pledged to Levinson's family, who have kept up a campaign to locate him for nearly two years.

The Levinsons live in Coral Springs, Fla. Nelson, a Democrat, is Florida's senior senator.

Levinson, 60, was last seen on March 8, 2007, on Kish Island, a tourist haven in the Persian Gulf, where he was doing investigative work for a private security firm. His credit cards and passport have not been used since his disappearance.

Iran says it has "no record" of Levinson's stay on Kish and has resisted attempts by the U.S. government to investigate.

The U.S. does not have formal diplomatic ties with Iran and uses the Swiss government as an intermediary. Iran has refused access to Swiss investigators seeking to visit Kish Island.

"Despite the grave differences we have with Iran, discussions with Tehran are vital to ensure the safe return of an American citizen," said Nelson. "Unfortunately, Iran continues to claim it has no information on the fate of Mr. Levinson."

Levinson's wife Christine flew to Iran with members of her family last December to search for clues to her husband's whereabouts.

She thanked Nelson Thursday for his tireless work to find her husband. "I want to express, for me and my children, our deepest gratitude to Sen. Bill Nelson and his caring staff for their continuing efforts to help us find Bob and bring him home," she said.

"It has now been 20 months since Bob — a wonderful husband and loving father — disappeared on Kish Island, Iran. We love and miss Bob more than words can say. We will never stop looking for him. We just want him home."

If Nelson's resolution passes, the Senate will join a growing chorus of voices calling for greater cooperation from Iran. The State Department has "[urged] Tehran to share any and all information the Iranian authorities have uncovered about the Levinson case," and the FBI has long been pushing for information on a man they call one of their own.

"This is a matter of great concern for the FBI. Bob had a long and distinguished FBI career, and he has a wife, four daughters, three sons, one grandchild, and another on the way, all awaiting his return. Plain and simple, our goal is to bring Bob home to his family," FBI Assistant Director Joseph Persichini Jr. said in July.

Thursday's call is the first public move the U.S. government has made in months to address Levinson's case. The resolution does not have the force of law and cannot impel the government to act on Levinson's behalf.

Outside of declarations of war, Senate resolutions generally have little impact in the U.S. Nelson's call to action is mostly a symbolic gesture, intended to increase pressure on the Iranian regime to account for Levinson.

"We cannot — and I believe I speak for the entire United States Senate when I say — we are not going to forget Bob Levinson and his family," Nelson said.

Anyone with information is urged to contact the U.S. State Department or the Levinson family at www.helpboblevinson.com.