A method of constructing a set of motifs for use in detecting messages of interest in a network of nodes is provided, the method comprising controlling circuitry to: acquire target data, the target data comprising a set of messages which have been exchanged between nodes in the network, the set of messages including a number of messages of interest; acquire control data, the control data comprising a set of messages which have been produced based on a random exchange of messages between nodes in the network; detect motifs within the target data and the control data, each motif being a repeated pattern of messages appearing within either the target data and/or the control data; generate a set of values indicative of a significance of the motifs which have been detected in the target data and the motifs which have been detected in the control data using a frequency with which these motifs have been detected; and construct a set of motifs for use in detecting messages of interest in the network using the set of values which have been produced and a comparison of the motifs detected in the target data with the motifs detected in the control data.