Secretary of State Hillary Rodham Clinton is pressing Iran to release Americans who are detained or have gone missing there, AP reported. Retired FBI agent Robert Levinson has been missing since March 2007. Three hikers _ Joshua Fattal, Shane Bauer and Sarah Shourd _ were detained by Iranian authorities on July 31. And an Iranian-American scholar, Kian Tajbakhsh, was arrested last month on charges related to provoking unrest. Clinton's statement Saturday comes days after a U.S. graduate student held in Iran returned to Los Angeles. The student, Esha Momeni, was imprisoned for a month because of her research on the Iranian women's rights movement. She was freed in November, but was only able to return to the U.S. this past week.