CAIRO/PARIS (Reuters) - A French naval vessel was en route to the eastern Mediterranean on Thursday to join the hunt for black boxes from a crashed EgyptAir jet, equipped with three specialist probes from a French company recruited to accelerate the search.

France's BEA air crash investigation agency said French naval survey vessel Laplace had left Corsica earlier on Thursday and was heading toward the search zone north of the Egyptian port of Alexandria, where it would begin operations within days.

A week after the Airbus A320 crashed with 66 people on board, including 30 Egyptians and 15 from France, investigators have no clear picture of its final moments.

But Egyptian investigators said a radio signal had been received from an emergency distress beacon usually located in the rear of the cabin. This could help narrow the search area for that part of the fuselage, near the tail where "black box" flight recorders are held, to a 5-km (3-mile) radius, they said.

The emergency locator transmitter (ELT) sends out a signal that can be picked up by satellites in the international search-and-rescue network when an aircraft is in an accident.

It is separate from the underwater locator beacons (ULB) or "pingers" attached to the "black box" flight recorders, which send out acoustic rather than radio signals and are designed to be more easily detected underwater.

John Cox, a former A320 pilot and chief executive of Washington-based Safety Operating Systems, expressed caution about the reported signal from the sunken wreckage.

"There is a low likelihood the ELT would survive and radio doesn't work as well as acoustic signals underwater," he said.

Search teams are working against the clock to recover the two flight recorders that will offer vital clues on the fate of flight 804, because the acoustic signals that help locate them in deep water cease transmitting after about 30 days.

The BEA, which is working as part of an Egyptian-led investigation into the crash, said two of its investigators were on board the French naval ship which was carrying equipment from ALSEAMAR, a firm specialising in searching for marine wrecks.

Negotiations are also under way to contract a second firm to search more than one area, French and Egyptian officials said.

ALSEAMAR's equipment includes three of its DETECTOR-6000 systems, designed to pick up black-box pinger signals over long distances up to 5 km (3 miles), according to the company's website.

It works by dipping a slender probe into the water to listen for pings and then retrieving it to download the findings.

ALSEAMAR, a subsidiary of French industrial group Alcen, did not respond to a request for comment.

In 2004, the same company deployed a system of "intelligent buoys" to search for black boxes after a Boeing 737 belonging to Egypt's Flash Air crashed in the Red Sea near Sharm al-Sheikh.

The second firm likely to be involved is Mauritius-based Deep Ocean Search, with which France and Egypt are finalising a contract, according to French diplomatic sources.

That firm was originally involved in the search for missing Malaysian jet MH370, but it and others voiced complaints about the conduct of the search after being rejected when responsibility shifted from Malaysia to Australia.

It was not immediately available for comment. 

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The EgyptAir black boxes are believed to be lying in up to 3,000 metres of Mediterranean water, on the edge of the usual range for picking up signals emitted by the boxes.

Maritime search experts say this means acoustic hydrophones are usually towed in the water at depths of up to 2,000 metres in order to have the best chance of hearing the signals.

Ayman al-Moqadem, Egypt's head of air accident investigations, said the investigating team had received radar imagery and audio recordings from Greece detailing the flight trajectory of the doomed plane and the last conversation between its pilot and Greek air traffic control.

It is expecting France to hand over radar imagery and other data covering the plane's time in French airspace and on the ground in Paris, he added.

Sources in the investigation committee have said the EgyptAir jet did not show technical problems before taking off from Paris. During flight, it sent signals that at first showed the engines were functioning but then detected smoke and suggested an increase in temperature at the co-pilot's window.

The plane kept transmitting messages for the next three minutes before vanishing.

With no flight recorders to check and only fragmentary data from a handful of fault messages including two smoke alarms, investigators are also looking to debris and body parts for clues.

Cox said the fault messages collectively pointed to a possible problem in the avionics bay under the cockpit, but stressed it was too early to rule out any possible cause.

Moqadem said no bodies had been recovered so far, with search teams only able to locate small body parts. DNA tests are underway to identify the remains.

He said a report would be issued by the investigating team one month from the date of the crash. 

(Additional reporting by John Irish, Ahmed Aboulenein, Editing by Ralph Boulton and Cynthia Osterman)