American-born, Iranian journalist held as ‘material witness,’ court documents show

The order confirms that Franklin has not been accused of any crime.

The U.S. government on Friday acknowledged for the first time that it secretly arrested an Iranian journalist earlier this week, demanding she testify to a federal grand jury as part of a federal probe that still remains a mystery.

The acknowledgement came in a federal judge's order stating that American-born Melanie Franklin, also known as Marzieh Hashemi, was taken into custody on a material witness warrant issued in Washington.

The order, unsealed Friday in federal court in Washington, D.C., confirms that Franklin has not been accused of any crime and has made two court appearances before a federal judge.

Franklin, the order states, is expected to be released immediately following the completion of her testimony before a federal grand jury.

This was also confirmed by her son to the Associated Press.

“We still have no idea what’s going on,” said Hossein Hashemi, who is Hashemi's son and who is a research fellow at the University of Colorado

Hasemi told the Associated Press that he and his siblings had been subpoenaed to appear before a grand jury.

In that same interview he said his mother had not been contacted by the FBI and would've "absolutely" cooperated.

Iranian state media had reported that the American-born Iranian journalist had been arrested in St. Louis on Sunday and transferred to Washington.

Iranian government officials have called for her release.

"The custody of Iran's reporter in the U.S. is highly political and she should be released immediately," Iranian Foreign Minister Mohammad Javad Zarif told Iranian state media, the Islamic Republic News Agency.

The order by Chief Judge Beryl Howell is the first acknowledgement by the U.S. government that she had been arrested and not charged.

Federal law permits the government to detain witnesses under court order in certain circumstances.

It is highly unusual to hold someone as a material witness and it remains unclear what the grand jury is investigating.

ABC News' Cindy Smith contributed to this report.