CAIRO, Egypt (AFP) – A French navy vessel equipped with deep-water listening devices has detected signals from one of the black boxes of the EgyptAir plane that crashed in the Mediterranean, investigators said Wednesday.

The flight recorders could contain crucial information to help solve the mystery of why the Airbus A320 plunged into the sea with 66 people on board en route from Paris to Cairo on May 19.

The signals were picked up by French survey ship Laplace which is using acoustic detection systems to listen for the “pings” emitted by the flight recorders, France’s aviation safety agency BEA said.

“The detection of this signal is a first step,” said BEA official Remi Jouty.

Egypt’s ministry of civil aviation had announced the potential breakthrough earlier, saying the signals were “assumed to be from one of the data recorders”.

Some of the wreckage has already been pulled from the Mediterranean along with passenger belongings. No survivors have been found.

Another vessel sent by Deep Ocean Search (DOS), a private company hired to help find the black boxes, is on its way to the area carrying a ship with a robot capable of diving up to 3,000 metres (yards) to retrieve the recorders.

The ship is due to arrive at the site within a week, the Egyptian ministry said.

“Extensive search efforts are being carried out to locate the two data recorders in preparation for their retrieval,” the ministry said.

The black boxes have enough battery power to emit signals for four or five weeks.

The French navy surveillance vessel Laplace which detected the signals is equipped with three devices capable of picking up the “pings” from the black boxes from a distance of up to five kilometres (3.1 miles).

Investigators have said it is too soon to determine what caused the disaster although a terror attack has not been ruled out.

France’s aviation safety agency has said the aircraft transmitted automated messages indicating smoke in the cabin and a fault in the flight control unit minutes before disappearing from radar screens.

The passengers were 30 Egyptians, 15 French citizens, two Iraqis, two Canadians, and citizens from Algeria, Belgium, Britain, Chad, Portugal, Saudi Arabia and Sudan. They included a boy and two babies.

Seven crew and three security personnel were also on board.

The crash comes after the bombing of a Russian airliner over Egypt’s restive Sinai Peninsula last October that killed all 224 people on board.

The Islamic State group claimed responsibility for that attack within hours, but there has been no such claim linked to the EgyptAir crash.

IS has been waging a deadly insurgency against Egyptian security forces and has claimed attacks in both France and Egypt.

In October, foreign governments issued travel warnings for Egypt and demanded a review of security at its airports after IS said it downed the Russian airliner over Sinai with a bomb concealed in a soda can that had been smuggled on the plane.