Esfandiari, who is an Iranian-born U.S. citizen, had traveled to Iran in December to visit her 93-year-old grandmother.
Iran confirmed the arrest of Esfandiari five days after she was detained in Tehran on unknown charges.
Foreign Ministry spokesman Mohammad Ali Hosseini told a news conference in Tehran on May 13 that the 67-year-old Esfandiari's arrest was based on Iranian law. He added that she would be treated like other Iranian nationals, and declined to say why Esfandiari was arrested or what charges she might face.
While it technically does not allow dual citizenship, Iran routinely turns a blind eye in issuing passports to Iranians living abroad.
Esfandiari, the director of the Woodrow Wilson International Center in Washington's Middle East program, was taken to Tehran's Evin prison on May 8. That facility is frequently used to house prisoners that the country's leadership regards as a possible threat, including its strongest critics.
On December 30, near the end of Esfandiari's visit to Iran, three masked and armed men stole her U.S. and Iranian passports. Instead of replacing her passport, Esfandiari was repeatedly summoned by intelligence officials for protracted interrogations. Questions reportedly centered on the activities and program of the Wilson center.
Serious Allegations
Esfandiari's arrest came amid a crackdown on Iranian activists, including women's rights advocates and students. It also comes at a time of heightened tensions between the United States and Iran.
Iranian officials have warned that foreign enemies are using Iranian activists and critics to instigate a "soft revolution" in Iran.
On May 12, Iranian state television used a commentary to accuse "media affiliated with the White House and some U.S. officials" of using the Esfandiari case as a " new project" to apply psychological pressure on Iran.
In a reference to Israel's intelligence and covert-operations agency, he commentary described Esfandiari as a "Mossad spy." He also claimed she has been active in efforts to create a "velvet coup d'etat" in Iran.
The hard-line daily "Kayhan" on May 12 accused Esfandiari of involvement in efforts to topple Iran's Islamic regime. It said she was a main element of Israeli efforts to spur revolution in Iran.
Esfandiari's husband, university professor Shaul Bakhash, has rejected the accusation against his wife as "false" and "hollow." In a statement to the media -- including Radio Farda -- Bakhash said it is "a lie" that his wife had "undercover assignments" or was a "media spy" in Iran.
Continuing Clampdown
Critics say the increased state pressure on Iranian intellectuals, journalists, and civil-society activists is an indication of the leadership's growing fear of a soft or color revolution. The U.S. allocation of $75 million to promote democracy in Iran has added to tensions.
Ramin Jahanbegloo was detained for several months last year (AFP file photo)