The study made use of eye-tracking technology to gain insight into the effects of guilty knowledge on eye movement. In addition, the study adds to existing literature by examining participants who possess guilty knowledge but are not guilty of the criminal event. The study used a completely randomized, between-subject design and consisted of three conditions: the bomb-building, the bomb-observing, and the puzzling condition. An experiment was conducted in which guilty knowledge was manipulated and required both bomb-building and bomb-observing participants to lie about recognizing an explosive device during a new version of Guilty Knowledge Test (GKT). Our findings show that it is possible to differentiate between guilty participants possessing guilty knowledge and participants without guilty knowledge by both eye gaze and fixation duration. Yet, it remains difficult to distinguish participants possessing guilty knowledge without being guilty from participants possessing guilty knowledge due to committing the crime. These findings are worrying with regards to false-positive outcomes. Future research should therefore start focusing on the eye movements of observers.