Abstract: A security seal fastener system includes a fastener, and an enclosure around a head of the fastener that provides visual evidence of access to the head after installation of the fastener. The enclosure may include a washer that is around a shaft of the fastener, and a cap that closes off an access opening in the washer. The cap has a spring that engages a groove in an inner surface of the washer, to lock the cap in place. A flange of the cap and a ridge of the washer prevent access to the spring. The washer has an anti-rotation feature to prevent rotation of the fastening system while the enclosure is intact. The cap has a knockout that can be perforated or removed to allow access to the head of the fastener, for removal of the fastener. The cap may have an identifier, such as a serial number.