This paper studies the moving target search problem in the context of multiple mobile sensor agents (MSA). A novel cooperative search formation, the progressively-spiral-in (PSI) algorithm, for a team of MSAs to search for a mobile target in an open field is proposed. Based on that, a sufficient condition in terms of the number MSAs for the existence of a non-escape search plan is derived. Simulation results are presented to demonstrate the effectiveness of the PSI algorithm given that the number of MSAs satisfies the sufficient condition.