Investigators have begun analysing the two severely damaged black boxes from EgyptAir flight MS804 as they seek to explain why the plane plunged into the sea, killing all 66 people on board.

It would require "lots of time and effort" to fix the two badly damaged black box recorders, sources on Egypt's Aircraft Accident Investigation Committee told Reuters on Sunday.

The committee said it started the analysis of the devices, with representatives from France and the United States, which are crucial to explaining why the Airbus A320 crashed on May 19 en route from Paris to Cairo.

The memory units from both the Cockpit Voice Recorder and the Flight Data Recorder were extracted from the devices and dried in a military facility for eight hours, the committee said in a statement.

Investigators are now conducting electrical tests on the memory units, the final step before trying to extract data.

If intact, the cockpit recorder should reveal pilot conversations and any cockpit alarms, as well as other clues such as engine noise. But crash experts say it may provide only limited insight into the reason for the crash.

With the data recorder, investigators have a greater chance of discovering the cause, provided its chip is still intact.

Investigators need to further analyse the memory units before deciding if they can be fixed locally or if they need to be sent abroad for repairs.

Search teams retrieved the Cockpit Voice Recorder on Thursday which they said was damaged but had the memory unit intact. They found the Flight Data Recorder on Friday.

While no explanation for the disaster has been ruled out, current and former aviation officials increasingly believe the root of the crash lies in the aircraft's technical systems, rather than deliberate sabotage.

There has been a series of airplane accidents at high altitude blamed on a cocktail of technical and pilot flaws.

The crash is the third blow since October to Egypt's travel industry, which is still suffering from the 2011 uprising that ended Hosni Mubarak's 30-year rule.

A Russian plane crashed in the Sinai Peninsula in October, killing all 224 people on board in an attack claimed by Islamic State. In March, an EgyptAir plane was hijacked by a man wearing a fake suicide belt. No one was hurt. – Reuters