Tehran, Iran -- A U.S. Navy veteran detained in Iran in July is being held in connection to a "private complaint," the semi-official Mehr news agency reported Friday. Michael White, 46, is the first known American to have been detained in Iran since Donald Trump became president. His family says he traveled to Iran to visit his girlfriend, who he met online, and was arbitrarily detained.

The Friday report quoted prosecutor Gholamali Sadeghi as saying the case is still "under investigation," without elaborating. Under Iranian law, a private complaint would refer to allegations made by a citizen, not the state. Sadeghi did not confirm reports that White faces security charges.

Sadeghi's vague explanation of White's detention, in which he notably did not confirm reports that White could be facing national security related charges, came a day after the U.S. freed Marzieh Hashemi, an American-born journalist who has worked for decades for Iranian state TV. She was released from U.S. custody after testifying before a grand jury in Washington, her representative said Wednesday. She was with family at a Washington-area hotel, a friend told CBS News.

Marzieh Hashemi YouTube/PressTV

Hashemi, an anchor for Iran's Press TV, was arrested by FBI agents at Lambert International Airport in St. Louis last week. She was then transported to Washington and jailed for 10 days. A court order unsealed Friday confirmed Hashemi was being held as a material witness in a criminal case. She was released without any charges on Thursday.

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White's arrest and fears for his health

White's mother, Joanne White, confirmed to CBS News early this month that her son never made his scheduled return flight from Iran, where he went to visit a girlfriend, at the end of July. The family reported him missing at that time, but only learned from the State Department in December that he was being held in an Iranian prison.

"I'm very worried about his health. He just got over cancer and I'm worried about his condition. It's very scary to me," she told CBS News. "He's been over there three or four times without any issues. The last time I had contact with him was July 13th."

White's mother said the family had not been informed of any charges against him. His family has called on the Trump administration to secure his release. Joanne White has said he was undergoing cancer treatment and feared he would not survive prolonged detention.

Michael White worked as a cook for the Navy and left the service about a decade ago, according to a spokesman for the family. Both his family and who insisted White was not a spy and had never been one. The spokesman, Jonathan Franks, said White had recently worked as a janitor.

President Trump has pursued a maximalist campaign against Tehran that includes pulling out of its nuclear deal with world powers and implementing punishing sanctions against the country.

Iran holding at least five Americans

There are at least four other known American citizens being held in Iran.

Iranian-American Siamak Namazi and his 82-year-old father Baquer are 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 years in prison for allegedly "infiltrating" the country while doing doctoral research on Iran's Qajar dynasty.

Iranian-American Robin Shahini was released on bail in 2017 after staging a hunger strike while serving an 18-year prison sentence for "collaboration with a hostile government." Shahini is believed to still be in Iran.

Also in an Iranian prison is Nizar Zakka, a U.S. permanent 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.

Sources tell CBS News that Iran is likely holding the Americans to try to extract concessions such as those received in a deal reached with President Obama. While the Trump administration offered to discuss prisoners last year, a senior administrative official said Iran turned that offer down, leaving the fate of the Americans unclear.

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, but his family holds Tehran responsible for his disappearance.