A novel technique for detecting multi-point attacks on an Industrial Control System (ICS) is described. The technique, referred to as Intermittent Control Actions (ICA), sends control signals intermittently to selected components to monitor the system using a process invariant. ICA was assessed experimentally for its effectiveness in an operational water treatment testbed. The experiments revealed (a) multi-point attack scenarios where ICA succeeds or fails to detect an attack, (b) issues in the design of key ICS components to ensure that the control actions in ICA do not lead to undesirable process behavior, and (c) constraints on the design of the physical system for safe use of ICA.