A spectrum sensing system for detecting illegal radio broadcast emissions is proposed using a centralized cooperative spectrum sensing technique based on a cognitive radio network architecture. Two possible decision making solutions are proposed. The first one consists of an energy detector is commonly used in cognitive radio systems, which is adapted to work under a changing threshold. This method takes into account the changing conditions of the illegal emissions. The second one is a peak detector used for finding carrier frequencies. Both solutions were tested for the FM radio broadcasting comparing their performance in terms of the receiver operating characteristic curve.