A forensic feature for a secure document comprises a base document layer and a covert material applied to the base document layer. The covert material includes a carrier and forensic material within the carrier. The forensic material includes a ratio of salts or oxides of metals, such as rare earth metals. The ratio is selected to correspond with a source of the document. The forensic material may be mixed into a coating or ink that is applied at predetermined locations on a secure document. The ratio is then measurable from metal ion signals of the salts or oxides. This ratio, or some metric derived from it, may be linked with information embedded elsewhere in the document to enable verification of the document. Another forensic document feature has a forensic metric that is measurable from a covert material in the document, and this forensic metric corresponds to a source of the document. A blocking layer applied over the covert material prevents access to the covert material such that at least partial destruction of the document is required to measure the forensic metric. The blocking layer may have a blocking property that blocks electromagnetic waves from activating the covert material, or blocks the electromagnetic waves from the covert material in response to the activating waves. The blocking layer is deconstructed to access the forensic feature, verify the document and perform forensic tracking.