The US Justice Department and the FBI have refused to comment on the detention of the Iranian Press TV host, CBS News reported. 

Marzieh Hashemi was arrested by the FBI during her visit to the US on Sunday. According to her son, she was being held in prison, apparently as a witness.

American-born Hashemi was detained at St. Louis Lambert International Airport in St. Louis, Missouri, on Sunday.

Hashemi has arrived in the US to visit her ill brother and other family members.

Both the FBI and the Justice Department have declined to comment on the case. 

"We still have no idea what's going on," said Hossein Hashemi, a research fellow at the University of Colorado whom the AP interviewed by phone from Washington.

According to him, he and his siblings should appear before a grand jury.

US federal law allows judges to arrest and detain witnesses if the government can prove their testimony has exceptional value for the criminal case and that they would be a flight risk and unlikely to respond to a subpoena

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.