A French ship searching the Mediterranean has detected black box signals from a missing EgyptAir flight in the waters between the Greek island of Crete and the Egyptian coast, a development that could help solve the mystery of why the aircraft crashed into the sea last month, killing all 66 on board. The discovery, announced Wednesday, could help guide search teams to the wreckage and the flight’s data and cockpit voice recorders, which if retrieved unharmed could reveal whether a mechanical fault or a hijacking or bomb caused the disaster. In the two weeks since Flight 804 disappeared from radar en route to Cairo from Paris, only small pieces of debris and human remains have been retrieved from the crash site. No terrorist group has claimed responsibility, though Egypt’s civil aviation minister, Sherif Fathi, has said terrorism is a more likely cause than equipment failure or some other catastrophic event.