Iran's judiciary has confirmed the detention of an Iranian-American who was visiting family in Iran, the country's semi-official ISNA news agency reported Sunday.

The report did not name the Iranian-American involved or say when he was arrested. It quoted the spokesman for the judiciary, Gholam Hossein Mohseni Ejehi, as saying that the man was arrested in the city of Gorgan on unknown charges and then referred to the Iranian capital for investigation.

On Thursday the U.S. State Department said it was looking into reports that Iranian-American Robin Shahini has been detained in Iran.

His girlfriend said she was worried Shahini was arrested over online comments criticizing Iran's human rights record. She said he was arrested in Gorgan, where he was visiting his family.

She said that Shahini's sister told her Iranian authorities took him into custody on July 11 and that he has not been heard from since.

Shahini, 46, left Iran in 1998 and lived in San Diego. He graduated in May from San Diego State University.

Iran does not recognize dual nationalities, which means he cannot receive consular assistance.

There are three dual nationals and a Lebanese man who have been detained in Iran in recent months. The four, who have ties to Britain, Canada and the United States, all are believed to have been detained by Iran's paramilitary Revolutionary Guard, a hard-line force charged with ensuring the country's Islamic government remains in power. The charges they face remain unclear.

In previous cases involving dual nationals, like the detention of Washington Post journalist Jason Rezaian, officials initially announced indictments had been handed down without providing specifics. Later, Iranian news organizations with close ties to security services offered details of the charges.

A prisoner swap in January between Iran and the U.S. freed Washington Post correspondent Jason Rezaian and three other Iranian-Americans.