The statement dashed hopes of any imminent breakthrough in the cases, after Iran announced last month that it was in the final stage of its probe and would announce whether the dual U.S.-Iranian nationals would be tried or freed within two or three days.
The two Americans are Haleh Esfandiari, a Potomac resident and director of the Middle East program at the Smithsonian's Woodrow Wilson International Center for Scholars, and Kian Tajbakhsh, a New York-based social scientist. Both have been charged with unspecified crimes against Iran's national security.
"We have received fresh evidence" about Esfandiari and Tajbakhsh, judiciary spokesman Alireza Jamshidi told reporters in Tehran. "Fresh investigations have started based on this evidence."
In Washington, the State Department and Esfandiari's husband called the new allegations unwarranted.
"It is obvious that the Ministry of Intelligence, lacking any real cause or evidence to keep my wife, Haleh, incarcerated in solitary confinement at Evin, is trying to drag things out by claiming continuing 'investigations,' " said George Mason University professor Shaul Bakhash, referring to a prison in Tehran.
In a further sign of Iran's clampdown, three Iranians who had been in the United States on a cultural exchange for documentary filmmakers had their passports confiscated upon their return to Tehran on Saturday and were instructed to report on Sunday to court, where they were told to await a further summons, according to U.S. officials. The State Department has declined to release the names of the three.
At least two other Americans are detained in Iran. California businessman Ali Shakeri was picked up on May 8, and correspondent Parnaz Azima of U.S.-funded Radio Farda has been unable to leave Iran for several months.