The mystery over the incarceration of a Navy veteran in Iran last July deepened on Friday, when an Iranian prosecutor said that the case had been based on an “individual plaintiff” and that the prisoner might face security-related charges.

The veteran, Michael R. White of Imperial Beach, Calif., is the first American to be imprisoned in Iran since the Trump administration took office two years ago.

His arrest raised the number of American prisoners in Iran to at least four, and has added an irritant to the already poor relations between the countries.

The prosecutor, Gholamali Sadeqi, in the northeast Iranian city of Mashhad, was quoted in a brief report by Iran’s Mehr news agency as saying “there is an individual plaintiff in the case,” but he did not provide details.