The U.S. State Department said Friday Iran's latest denial of knowledge in the case of missing former FBI agent Robert Levinson lacks credibility. Levinson disappeared after arriving at an Iranian beach resort in March. VOA's David Gollust reports from the State Department.

The Levinson case has spawned rare diplomatic exchanges between the U.S. and Iranian governments through their Swiss intermediary, but officials here remain skeptical about Iran's insistence that it has no information about him.

The 59-year-old Levinson, who retired from the FBI several years ago, went missing after arriving at Iran's Kish island in the Persian Gulf, reportedly to do research for a documentary film.

The State department has sent several inquiries about Levinson to the Iranian government through the Swiss, who look after U.S. interests in Iran in the absence of diplomatic relations.

In a talk with reporters, State Department Deputy Spokesman Tom Casey said a new Iranian message was received here Wednesday and, in it, the Tehran government again denied any knowledge of Levinson's whereabouts.

Casey said U.S. officials are sure Levinson arrived in Iran and have no information that he left, and he said that leads them to believe that Iranian authorities should be able to find out something about the case.

"We believe that there are many possible leads out there for them to pursue, including a number of things that have been reported on in the press in the past," said Casey. "So we're going to continue to be pursuing this, but unfortunately the Iranian government's response to date on this has, again, to simply say they have no information. And we just don't think that's very credible."

Officials say that in the messages to Iran, the State Department has cited local press accounts of Levinson's apparent arrest. The United States has also asked three friendly countries that have relations with Iran to inquire about Levinson, but Casey said that channel has also been unproductive.

Casey again urged Iran to end what he called the unjustified detention of four Iranian-born American citizens who have either been arrested in Iran in recent weeks or been barred from leaving that country after paying visits.

The appeal came after Iran confirmed for the first time that it is holding California businessman Ali Shakeri, a board member of the University of California at Irvine's Center for Citizen Peacebuilding.

The government-affiliated Iranian Student News Agency said Shakeri, who was supposed to have left Iran for Europe a month ago, was under investigation on security-related charges.

Iran has also jailed Iranian-American academics Haleh Esfandiari and Kian Tajbakhsh on spy charges and is preventing journalist Parnaz Azimah of the U.S.-funded Radio Farda from leaving the country after a family visit.

Casey reiterated that the four were not in Iran on any U.S. government-related business and that Tehran should "do the right thing" and allow them to return home:

"It's clear to all of us that regardless of the differences between the governments of our two countries, that on a basic human level, no one should be faced with imprisonment or fear of harassment simply because they want try and go see their family or because they want to try and stimulate the kind of personal contacts and people-to-people contacts that are pretty much normal, standard interactions," he said.

The State Department late Thursday issued a statement expressing dismay over what it said was Iran's decision to harass and in some cases imprison American citizens on "groundless charges."

President Bush last week called for the immediate and unconditional release of the dual nationals, while saying he was "disturbed" by Tehran's refusal to provide information about Levinson.