CAIRO (AP) — Smoke was detected in multiple places on EgyptAir flight 804 moments before it plummeted into the Mediterranean, but the cause of the crash that killed all 66 on board remains unclear, the French air accident investigation agency said on Saturday.

The Egyptian military has shared some of the first photos of the wreckage from EgyptAir flight 804. The images, which show a life jacket, part of a chair, and what appears to be a ruined section of fuselage, come after French investigators confirmed on Saturday that smoke was detected in multiple places onboard the aircraft moments before it plummeted into the Mediterranean. (Scroll down for video of the wreckage.)

Despite this development, the cause of the crash that killed all 66 on board remains unclear.

Agency spokesman Sebastien Barthe told The Associated Press in Paris that the plane's automatic-detection system sent messages indicating smoke a few minutes before the plane disappeared from radar while flying over the east Mediterranean early on Thursday morning.

The messages, he explained, "generally mean the start of a fire," but he added: "We are drawing no conclusions from this. Everything else is pure conjecture."

The industry publication Aviation Herald also reported that sensors detected smoke in the plane's lavatory, suggesting a fire onboard.

A life jacket believed to have originated on the doomed EgyptAir flight. Egyptian Army

Looking for clues to whether terrorists may have brought down the Airbus A320, investigators have been poring over the passenger list and questioned ground-crew members at Charles de Gaulle Airport in Paris, from which the plane took off.

The aircraft had been cruising normally in clear skies on a nighttime flight to Cairo early Thursday when it suddenly lurched left, then right, spun all the way around and plummeted 38,000 feet (11,582.4 meters) into the sea, never issuing a distress signal.

Search crews, meanwhile, are scouring for further wreckage of the aircraft. Experts said answers will come only with an examination of the wreckage and the plane's cockpit voice and flight-data recorders, commonly known as black boxes.

Planes and vessels from Egypt and five other countries — Greece, Britain, France, the US, and Cyprus — continued searching a wide area of the eastern Mediterranean on Saturday, a day after the Egyptian military found debris from the passenger jet in the sea 180 miles (290 kilometers) north of the Egyptian port city of Alexandria.

The waters in the area are 8,000 to 10,000 feet deep (2,440 to 3,050 meters), and the pings from the black boxes can be detected up to a depth of 20,000 feet (6 kilometers).

What appears to be part of the fuselage from the EgyptAir flight. Egyptian Army

On Saturday, the Egyptian military posted new photos of debris on its spokesman's Facebook page. They included what appeared to be parts of a seat, a life jacket, and what looks to be a pink and purple children's blanket.

Egyptian authorities said they believe terrorism is a more likely explanation than equipment failure, and some aviation experts have said the erratic flight suggests a bomb blast or a struggle in the cockpit. But so far no hard evidence has emerged.

No militant group has claimed to have brought down the aircraft. That is a contrast to the downing of a Russian jet in October over Egypt's Sinai Peninsula that killed all 224 on board. In that case, the Islamic State group's branch in Sinai issued a claim of responsibility within hours. On Friday, IS issued a statement on clashes with the Egyptian military in Sinai, but said nothing about the plane crash.

Three European security officials said on Friday that the passenger manifest for Flight 804 contained no names on terrorism watch lists. The officials spoke on condition of anonymity because they were not authorized to discuss the investigation. The manifest was leaked online and has not been verified by the airline.

Further checks are being conducted on relatives of the passengers.

More of the wreckage from the crashed plane. Egyptian Army

French aviation investigators have begun to check and question all baggage handlers, maintenance workers, gate agents, and other ground-crew members at Charles de Gaulle Airport who had a direct or indirect link to the plane before it took off, according to a French judicial official. The official was not authorized to discuss the investigation and spoke on condition of anonymity.

Whatever caused the aircraft to crash, the tragedy will very likely deepen Egypt's difficult predicament as the country of some 90 million people struggles to revive a battered economy and contain an increasingly resilient insurgency by Islamic militants.

In the five years since the country was first mired in turmoil caused by a popular uprising against longstanding dictator Hosni Mubarak, the lucrative tourism sector has been decimated, the value of the local currency has progressively fallen, and domestic debt has mushroomed.

While the latest tragedy may not reflect directly on security at Egypt's airports, as the Russian jet crash did in October, the country's association with another air disaster will negatively impact tourism and the flow of foreign investment.

Part of a chair from the EgyptAir flight discovered by investigators. Egyptian Army

In Paris, French Foreign Minister Jean-Marc Ayrault was meeting Saturday with family members of the passengers and crew aboard the EgyptAir flight.

Ayrault, who is being joined by diplomats and investigators, said Friday the meeting at the Foreign Ministry would be aimed at giving family members as much information as possible.

"The whole truth, nothing but the truth," he was quoted as saying by broadcaster BFMTV. "We owe it to the families."

Here is video footage of the wreckage recovered: