there is probable cause to believe that the person possessing such materials has committed or is committing the criminal offense to which the materials relate: Provided, however, That a government officer or employee may not search for or seize such materials under the provisions of this paragraph if the offense to which the materials relate consists of the receipt, possession, communication, or withholding of such materials or the information contained therein (but such a search or seizure may be conducted under the provisions of this paragraph if the offense consists of the receipt, possession, or communication of information relating to the national defense, classified information, or restricted data under the provisions of section 793, 794, 797, or 798 of title 18, or section 2274, 2275, or 2277 of this title, or section 783 of title 50, or if the offense involves the production, possession, receipt, mailing, sale, distribution, shipment, or transportation of child pornography, the sexual exploitation of children, or the sale or purchase of children under section 2251, 2251A, 2252, or 2252A of title 18); or