A French naval vessel fitted with sophisticated underwater sensors has picked up a signal believed to have emanated from one of two data recorders from EgyptAir Flight 804, Egyptian and French officials said on Wednesday.

The flight crashed into the Mediterranean Sea on May 19 while en route to Cairo from Paris, killing all 66 people on board. Data signals, and the discovery of wreckage and human remains, have suggested a rapid loss of control just before the crash.

But the cause of the disaster remains a mystery. The flight recorders will be critical for determining whether the crash was the result of a deliberate act or an accident.

The vessel that picked up the signal, the survey ship Laplace, has been taking part in the search for the data recorders from the Airbus A320 since last week.

Another research vessel, the John Lethbridge, is being prepared to join the search team within a week and to retrieve the recorders if they are found, officials said. The vessel is operated by Deep Open Search, a company based in Mauritius.

The search for the EgyptAir plane has narrowed to an area within a five-kilometer radius of the point in the Mediterranean where satellite data indicate that the plane probably went down.

Provided the black boxes are undamaged, the information they contain will help crash investigators piece together the cause of the disaster.