Investigators are subjecting the flight recorders from the EgyptAir jet which crashed last month to electrical tests before analysis can begin.
Egyptian air accident investigation sources told Reuters news agency it would take “lots of time and effort” to fix the two damaged recorders.
The flight data and cockpit voice recorders were retrieved this week.
It is still unknown why Flight MS804 crashed into the Mediterranean on 19 May, killing all 66 people on board.
The plane was flying from Paris to Cairo when it vanished from radar.
Investigators have said it is too early to rule out any causes for the crash, including terrorism.
The recorders are being tested in the Egyptian civil aviation ministry’s laboratories in the capital Cairo, AFP news agency reports.
Egypt’s Aircraft Accident Investigation Committee will analyse them along with representatives from France and the US.
The work may take several weeks. Depending on the scale of the damage, the “black boxes” may be sent abroad for repairs.
Earlier, the committee said the data recorder had been retrieved “in several pieces” by a specialist ship, the John Lethbridge, which found the wreckage on Wednesday in several locations about 290km (180 miles) north of the Egyptian coast, at a depth of about 3,000m (9,800ft).
Egypt’s civil aviation minister has said a terrorist attack is more likely than a technical failure.
Automated electronic messages sent by the plane revealed that smoke detectors had gone off in a toilet and in the avionics area below the cockpit, minutes before the plane’s signal was lost.
On Monday, the investigation committee confirmed that radar data had shown the plane turned 90 degrees left and then 360 degrees to the right, dropping from 11,300m (37,000ft) to 4,600m (15,000ft) and then 3,000m (10,000ft) before it disappeared.
What do we know so far?
- EgyptAir Flight MS804 vanished over the eastern Mediterranean early on Thursday 19 May with 66 passengers and crew on board
- Some surface debris was found 290km (180 miles) north of the Egyptian city of Alexandria
- Wreckage was subsequently found in several locations at a depth of about 3,000m (6,800ft)
- Signals from the plane indicated that smoke was detected in the toilet and in the avionics area below the cockpit
- Aircraft made a 90-degree left turn followed by a 360-degree turn to the right before vanishing off radar