A U.S. freelance journalist detained about a month ago is being held in a prison north of Tehran, Iran's judiciary spokesman said.

Ali Reza Jamshidi says 31-year-old Roxana Saberi is being held in Evin prison on a court order. He refused to provide further details in Tuesday's press conference.

An Iranian-American journalist, whose family has not heard from her for three weeks, was arrested for engaging in “illegal” activities because she continued to work after the government revoked her press credentials Iran said Monday.

Roxana Saberi moved to Iran six years ago and had previously reported for NPR, the BBC and FOX News.

"She called from an unknown place and said she’s been kept in detention,” her father Reza Saberi said about the Feb. 10 call. “She said that she had bought a bottle of wine and the person that sold it had reported it and then they came and arrested her."

The wine purchase was just an excuse to arrest her, he said.

Roxana Saberi said that she had already been held for ten days, and called back moments later to say that she would be released in two more days, the Times of London reported.

Neither her family in the U.S. nor her friends in Tehran have heard from her since.

Iran’s Foreign Ministry, which confirmed Rozana Saberi’s arrest, did not say why her press credentials had been revoked in 2006.

“Her accreditation was over in 2006 after Iranian authorities revoked her press card,” Hasan Qashqavi, a spokesman said Monday. “Her activities since 2006 were completely illegal and unauthorized.”

The Associated Press contributed to this report.