Iran's Foreign Ministry spokesman said Sunday that his country has been searching for a former FBI agent missing in Iran for almost a month.

Robert Levinson, a U.S. citizen who retired from the FBI in 1998, was last seen March 11 on the Iranian resort island of Kish. He was said to be working on a film on the island, known for its beaches, sea turtles and relatively liberal atmosphere.

"Authorities are pursuing to find more clues," said Mohammad Ali Hosseini during his weekly news briefing. "God willing, we will find definite information about his fate."

Hosseini said Iran had been in contact with the Swiss Embassy in Tehran, which houses a U.S. interests section, but still needed more details about the missing agent, who worked in New York and Florida and was known for busting Italian and Russian mobsters.

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Washington broke its diplomatic ties with Tehran after Iranian militant students stormed its embassy in Tehran in 1979 and kept its occupants as hostages for 444 days.

Levinson's relatives said last week they are worried and doing everything possible to find him. U.S. officials, meanwhile, have downplayed the disappearance as routine.

Americans, as well as nationals of other countries, need no visa to enter Kish, one of three Iranian free-trade zones with relaxed visa and trade rules.