Iran's judiciary said on Monday that Yeganeh Salehi, a foreign correspondent for The National, is being held for "security issues", The National reported.
The statement from judiciary spokesperson Gholam Hossein Mohseni Ejehi is the first indication of why Ms Salehi was arrested in Tehran on June 22 along with her husband Jason Rezaian, a correspondent for the Washington Post.
Mr Ejehi said the case was still in the "initial stages of investigation."
Two photographers were arrested at the same time as Ms Salehi. One of the photographers was later released. The families' of the photographers have asked that their identies are not released.
"The reason behind their detention is not financial but security issues," Mr Ejehi said, according to reports.
"In line with the law I cannot reveal the details of the case or the charges facing the accused."
Ms Salehi is an Iranian citizen while Mr Rezaian holds dual United States-Iranian citizenship.