Iran has protested the arrest of Marzieh Hashemi, who was born in the United States and is a presenter on Iran's English-language Press TV, amid high tensions between the two countries.
At a hearing in Washington, Beryl Howell, the chief judge of the US District Court for the District of Columbia, ordered the partial unsealing of an order on Hashemi, referred to by her former name of Melanie Franklin.
The order said that Hashemi was arrested on "a material arrest warrant" in a case in Washington.
She will be released immediately after the completion of her testimony to a grand jury investigating unspecified "violations of US criminal law."
"Franklin has been appointed an attorney and has not been accused of any crime," it said.
Hashemi married an Iranian man and converted to Islam. She has produced documentaries critical of US policy in the Middle East and the country's treatment of Muslims and African Americans.
She was arrested on Sunday at the St. Louis airport on a visit to see an ailing family member, Press TV said.
Iran, generally on the receiving end of criticism by the United States on press freedom and human rights, pledged to defend Hashemi, who it considers an Iranian citizen due to her marriage.
"The arrest of Marzieh Hashemi by America is an unacceptable political act that tramples on freedom of speech," Foreign Minister Mohammad Javad Zarif said on Wednesday.
The New York-based Committee to Protest Journalists voiced concern earlier on Friday about the arrest, saying that the United States needed to disclose the reason for her arrest.
The advocacy group said that Iran is detaining at least eight journalists over their work.
In one of the most notorious cases, Iran detained Washington Post journalist Jason Rezaian for 544 days before he was freed as part of a reconciliation deal with President Barack Obama's administration.
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