In this paper a power system state estimator cyber-attack detection and identification scheme is presented. The proposed scheme considers the information from both the active power measurements and the reactive power measurements. Under the scenario that the network operator can take multiple different samples of measurements and the attackers can attack only once, the proposed scheme can detect the presence of what has been described as stealth (or unobservable) attacks. The detection is provably correct. Furthermore, if an attack is present, the proposed scheme can identify exactly the attacked transmission lines.