Marzieh Hashemi, who worked for the network's English-language service, was detained in St. Louis, where she had filmed a Black Lives Matter documentary after visiting relatives in the New Orleans area. She was then taken to Washington, according to her elder son, Hossein Hashemi.
The FBI said in an email that it had no comment on the arrest of the woman who was born Melanie Franklin in New Orleans and has worked for Iran's state television network for 25 years.
Hossein Hashemi said his mother lives in Tehran and comes back to this country about once a year to see her family, usually scheduling documentary work somewhere in the U.S. as well.
"We still have no idea what's going on," said Hashemi, a research fellow at the University of Colorado who was interviewed by phone from Washington. He also said he and his siblings had been subpoenaed to appear before a grand jury.
The incident comes as Iran faces increasing criticism of its own arrests of dual citizens and other people with Western ties. Those cases have previously been used as bargaining chips in negotiations with world powers.
Federal law allows judges to order witnesses to be arrested and detained if the government can prove their testimony has extraordinary value for a criminal case and that they would be a flight risk and unlikely to respond to a subpoena. The statute generally requires those witnesses to be promptly released once they are deposed.
Marzieh Hashemi, an American citizen, had not been contacted by the FBI before she was detained and would "absolutely" have been willing to cooperate with the agency, her son said.
Asked whether his mother had been involved in any criminal activity or knew anyone who might be implicated in a crime, Hashemi said, "We don't have any information along those lines."
Hashemi said his mother was arrested as she was about to board a flight from St. Louis to Denver. A spokesman for St. Louis Lambert International Airport declined to comment and referred questions to the FBI.
The constitutionality of the material witness law has "never been meaningfully tested," said Ricardo J. Bascuas, a professor at the University of Miami School of Law. "The government only relies on it when they need a reason to arrest somebody but they don't have one."
No matter the reason for Marzieh Hashemi's detention, she should have been granted a court appearance by now, Bascuas said.
She apparently was unable to call her daughter until Tuesday night. The family is trying to hire an attorney, but it has been difficult because she has not been charged with a crime, her son said.
Iran's state broadcaster held a news conference and launched a hashtag campaign for Hashemi, using the same techniques families with loved ones held in the Islamic Republic use to highlight their cases.
"We will not spare any legal action" to help her, said Paiman Jebeli, deputy chief of Iran's state IRIB broadcaster. Iran's Press TV aired footage of her anchoring news programs and discussing the war in Syria, set to dramatic music.
There were no references to any case against Hashemi in U.S. federal courts, nor in Missouri.
Hashemi describes herself online as having studied journalism at Louisiana State University in Baton Rouge. She converted to Islam in 1982 at age 22 after meeting Iranian activist students in Denver.
She married a man she met while in journalism school. They had two sons and a daughter. Her husband is dead, said Hashemi's brother, Milton Leroy Franklin of the New Orleans suburb of Metairie.
Last week, Iran confirmed it is holding U.S. Navy veteran Michael R. White at a prison, making him the first American known to be detained under President Donald Trump's administration.
Iranian Foreign Ministry spokesman Bahram Ghasemi told state TV that Hashemi's arrest indicates the "apartheid and racist policy" of the Trump administration.
"We hope that the innocent person will be released without any condition," Ghasemi said.
At least four other American citizens are being held in Iran, including Iranian-American Siamak Namazi and his 82-year-old father, Baquer, both serving 10-year sentences on espionage charges. Iranian-American art dealer Karan Vafadari and his Iranian wife, Afarin Neyssari, received 27-year and 16-year prison sentences, respectively. Chinese-American graduate student Xiyue Wang was sentenced to 10 year in prison.
Also in an Iranian prison is Nizar Zakka, a permanent U.S. resident from Lebanon who advocated for internet freedom and has done work for the U.S. government. He was sentenced to 10 years on espionage-related charges.
Former FBI agent Robert Levinson, who vanished in Iran in 2007 while on an unauthorized CIA mission, remains missing as well. Iran says that Levinson is not in the country and that it has no further information about him. His family holds Tehran responsible for his disappearance.