CAIRO — The search for the EgyptAir plane which crashed last week killing all 66 people on board has narrowed to a 3-mile-wide area in the Mediterranean Sea, based on signals from the craft’s emergency beacon, Egypt’s chief investigator said. The ELT’s signal is too weak to transmit information from underwater, unlike the locator pings emitted by the flight data and cockpit voice recorders, known as the black box. A French naval oceanographic research ship, Laplace, carrying a long-range acoustic system able to detect signals from the black box is headed to the crash site, France’s air accident investigation agency, the BEA, said in a statement.