(CNN) Marzieh Hashemi, an American anchor working for Iran's state television, is being held in the United States as a material witness and has not been accused of any crimes, a court document shows.

Hashemi, a prominent anchor for Iranian state-funded Press TV, was arrested Sunday at the St. Louis international airport, where she was filming a documentary on Black Lives Matter, the Iranian broadcaster and her family said.

The woman, who was born in the US as Melanie Franklin, is expected to be released from a Washington, D.C. detention facility after she testifies before "a grand jury investigating violations of U.S. criminal law," the court document released Friday said.

It is not immediately clear what case Hashemi's testimony is of interest.

Hashemi has been appointed an attorney and has made two appearances in federal court since her arrest, the document noted.

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