Water Distribution Networks (WDNs) are an integral part of society. Deliberate introduction of chemical or biological agents through accessible sites of a WDN can spread through the entire system and cause widespread damage to public health. In order to protect against such deliberate attacks on a WDN, an effective and efficient online monitoring system through sensors is needed. It is clear that sensors located at different nodes respond at different times depending on which vulnerable node is attacked. In the present study, we design sensor networks for contamination detection and identification which exploit the differences in sensor response times as additional information. A hydraulic analysis of the network is first carried out for a given loading condition to determine the flow directions and flow velocities in different pipes. Directed paths between vulnerable nodes and potential sensor nodes are used to construct a bipartite graph, and the sensor network design problem is formulated as a minimum set cover problem. Algorithms based on greedy heuristics are used to solve the set cover problem and obtain the corresponding sensor network. The proposed method is applied on two WDNs, and the use of sensor response times to obtain a design with reduced number of sensors is demonstrated. © 2016, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.