Industrial control systems are a vital part of the critical infrastructure. The potentially large impact of a failure makes them attractive targets for adversaries. Unfortunately, simplistic approaches to intrusion detection using protocol analysis or naïve statistical estimation techniques are inadequate in the face of skilled adversaries who can hide their presence with the appearance of legitimate actions.This paper describes an approach for identifying malicious activity that involves the use of a path authentication mechanism in combination with state estimation for anomaly detection. The approach provides the ability to reason conjointly over computational structures, and operations and physical states. The well-known Tennessee Eastman reference problem is used to illustrate the efficacy of the approach.