An American-born journalist who works for an Iranian state-run television channel was arrested in the United States as a material witness in an undisclosed investigation, according to a federal court order unsealed on Friday.

The journalist, Marzieh Hashemi, was arrested at the St. Louis airport on Sunday and was transferred by the F.B.I. to Washington, according to Press TV, where she works as an anchor.

American officials had not confirmed her detention until Friday, when Chief Judge Beryl A. Howell of the United States District Court for the District of Columbia ordered some details about her arrest unsealed.

The court order said that Ms. Hashemi, 59, had been arrested on a material witness warrant and was not accused of any crime. She had made two court appearances, it said, and American authorities expected her to be released after “her testimony before a grand jury investigating violations of U.S. criminal law.” Though the order said a lawyer had been appointed for Ms. Hashemi, none was named.