The Iranian lawyer of a U.S. citizen, whose son has been in custody in Iran for the past four months, says his client was detained upon returning from abroad.

Attorney Mahmoud Alizadeh Tabatabei told The Associated Press on Thursday that he has not been able to make contact with his client, Baquer Namazi, despite his efforts and was told by authorities that he would "be informed later" about the case.

He says the ailing, 80-year-old Namazi is being held in Tehran's Evin prison.

Tabatabei says he believes it is unlikely Namazi, who was detained on Monday, will face "political charges" but that he is probably "detained "for some investigation only."

Both Baquer Namazi and his son Siamak Namazi, detained since October in Iran, are dual U.S.-Iranian citizens.