Abstract: An attempted lock break-in detector of the invention comprising: a cover (1) provided with mounting means (2) for mounting on a door (3) provided with a lock (4), said cover (1) being capable of passing from a first position in which it covers means (40) for opening or closing said lock (4) so as to make said means inaccessible, to a second position in which it allows access to said opening or closing means (40), and vice versa; and means (51, 52) for opening and closing an electric circuit (6) to generate and interrupt an electrical signal (S1, S2) indicative of said cover (1) passing from the first position to the second position, and vice versa; said means (51, 52) for opening and closing an electric circuit (6) comprise at least one contact element (51) for making contact with an element (41) of said lock (4), which lock element constitutes a section of said electric circuit (6). The invention also provides a lock, a door, and an alarm system including such a detector.