President Bush has strongly condemned Iran's detention of four Iranian-Americans, and called for them to be freed "immediately and unconditionally."
Mr. Bush said in a statement Friday that the four - peace activist Ali Shakeri, scholars Haleh Esfandiari and Kian Tajbakhsh, and journalist Parnaz Azima - pose no threat to Iran. He noted that they went to Iran either to visit their parents or conduct humanitarian work.
The president also demanded to know the whereabouts of a former FBI agent, Robert Levinson, who has been missing in Iran since early March.
Thursday, the U.S. State Department confirmed that Shakeri is being detained at Tehran's Evin prison. It is not clear if Shakeri has been formally charged.
Academics Haleh Esfandiari and Kian Tajbakhsh are also being held in the prison and have been charged with spying.
Journalist Azima is also charged with spying, and has been barred from leaving Iran.
Shakeri is an advisory board member at a non-governmental organization, the Center for Citizen Peacebuilding at the University of California at Irvine.
Esfandiari is the director of the Middle East Program at the Woodrow Wilson International Center for Scholars, a Washington-based foundation.
Tajbakhsh is an urban planning consultant linked to the New York City-based Open Society Institute. Billionaire businessman George Soros established the institute to promote democracy and human rights.
Azima works for U.S.-funded Radio Farda. Her passport was confiscated, and she has been barred from leaving Iran since January.
Some information for this report was provided by AFP and Reuters.