Iran is investigating a fourth Iranian-American who has been held in the Islamic Republic, the country's semiofficial news agency ISNA reported Friday.

ISNA reported that Ali Shakeri was in custody, and his case was being studied in the security department of Tehran's prosecutor office. The news agency did not elaborate.

The report was the first Iranian confirmation that Shakeri, who is a founding board member of the University of California, Irvine, Center for Citizen Peace building, was being investigated. The U.S. State Department had said Shakeri, who was supposed to have left Iran for Europe on May 13 but never arrived at his destination, was being held at a notorious prison in Tehran.

ISNA was the only Iranian news agency on Friday to report that Shakeri was being investigated. Calls to Iranian judicial officials were not immediately returned Friday, which is the weekend in the Islamic country.

The semiofficial news agency is often used by Iranian officials for leaking information and testing public opinion reaction to sensitive cases.

The other three Iranian-Americans held in Iran and accused of espionage are: scholar Haleh Esfandiari, who is the director of the Middle East program at the Woodrow Wilson International Center for Scholars, the other two Iranian-Americans accused of espionage are: Kian Tajbakhsh, an urban planning consultant with George Soros' Open Society Institute; and journalist Parnaz Azima, who works for U.S. funded-Radio Farda.

The three have been charged with endangering Iran's national security and espionage, according to Iran's judiciary spokesman. It was not immediately known if Shakeri was also charged.

All were in Iran visiting family members or engaged in professional work, according to the State Department and their relatives and employers.

President Bush had demanded that Iran "immediately and unconditionally" release the four and has denied that they were spying for the U.S. Family, colleagues and employers also have denied the allegations.

Bush's remarks drew sharp criticism from Iranian officials. Earlier this week, Iran accused Bush of interfering in the country's internal affairs.

Iran in recent weeks has escalated accusations against the U.S., saying it had uncovered spy rings organized by the U.S. and its Western allies.

The State Department has warned U.S. citizens against traveling to Iran, accusing Iranian authorities of a "disturbing pattern" of harassment of Iranian-Americans.

The U.S. has also expressed concern about former FBI agent Robert Levinson, who went missing in Iran while on private business there in March.