CAIRO — Egypt said Thursday it has recovered the cockpit voice recorder from the submerged wreckage of EgyptAir Flight 804, a major breakthrough in the investigation that could help resolve the mystery of why the jetliner plunged into the Mediterranean last month and killed all 66 people aboard.

The announcement came a day after officials said they had found the wreckage of the Airbus A320 and are putting together a map of the debris on the seabed. Such images will help investigators determine whether the plane broke apart in the air or stayed intact until it struck the water, aviation experts said.

The wreckage of the Paris-to-Cairo flight is believed to be at a depth of about 9,800 feet. Previously, search crews found only small floating pieces of debris and some human remains.

Although designed to survive a crash and fire, the recorder had sustained damage, and only its memory unit – “the most important in the recorder” – was recovered unharmed, it said, without elaborating on the extent of the damage.