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A stele and flowers laid in memory of the victims are placed in the area where the Germanwings jetliner crashed in the French Alps, in  Le Vernet, France, Friday, March 27, 2015. The crash of Germanwings Flight 9525 into an Alpine mountain, which killed all 150 people aboard, has raised questions about the mental state of the co-pilot. Authorities believe the 27-year-old German deliberately sought to destroy the Airbus A320 as it flew Tuesday from Barcelona to Duesseldorf. (AP Photo/Christophe Ena)
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SEYNE, FRANCE - MARCH 25: (Alternate crop of #467495310) In this handout image supplied by the Ministere de l'Interieur (French Interior Ministry), search and rescue teams attend to the crash site of the Germanwings Airbus in the French Alps on March 25, 2015 near Seyne, France. Germanwings flight 4U9525 from Barcelona to Duesseldorf has crashed in Southern French Alps. All 150 passengers and crew are thought to have died. (Photo by F. Balsamo/Gendarmerie nationale/Ministere de l'Interieur via Getty Images)
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CNN —  

Families are grieving. Flight crews are in disbelief. Entire countries are in mourning.

That much is clear. But much else about Germanwings Flight 9525, which crashed Tuesday in the southern French Alps, is not.

In a disturbing development Thursday, a French prosecutor said that audio from the mangled voice recorder of Germanwings Flight 9525 reveals the captain was locked out of the cockpit while co-pilot Andreas Lubitz appeared to make a deliberate attempt to destroy the plane.

That revelation brings investigators closer to pinpointing who was responsible for the crash, but major questions remain. Chief among them: Why would the plane’s co-pilot crash the plane, and how did it happen?

Here’s the key information that’s available so far, and the big questions that remain.

The flight

Flight 9525 – operated by Germanwings, a low-cost division of Lufthansa – took off at 10:01 a.m. Tuesday from Barcelona, Spain, bound for Dusseldorf, Germany. The plane had 144 passengers and six crew members.

According to French aviation accident investigators, the plane began descending from its cruising altitude of 38,000 feet at 10:31 a.m. It lost contact with French radar at an altitude of 6,175 feet at 10:40 a.m., the investigators said.

The plane’s descent, French and German investigators say, started when Lubitz was at the controls.

New data about the flight released Thursday appeared to support authorities’ claim that someone set the plane on a crash course.

Transponder data shows that the autopilot was reprogrammed by someone inside the cockpit to change the plane’s altitude from 38,000 feet to 100 feet, according to Flightradar24, a website that tracks aviation data.

The aircraft crashed in a remote area near Digne-les-Bains in the Alpes de Haute-Provence region. All aboard are presumed dead.

The big questions: If Lubitz brought the plane down, as it appears, why? And if the co-pilot’s goal was to crash the plane, why wouldn’t he send it into a more rapid descent?

The final moments

Air traffic controllers sent out a distress call after radio contact was lost, but the plane’s crew didn’t respond or issue a distress call of their own, the French Civil Aviation Authority said.

Audio from the plane’s cockpit voice recorder includes normal exchanges between the captain and co-pilot at the beginning of the flight, according Brice Robin, the Marseille prosecutor.

But after the captain leaves to go to the bathroom, something changes. The recording picks up audio of the captain, who’s somehow locked out of the cockpit, trying to get back in, Robin said.

Carsten Spohr, the CEO of Germanwings parent company Lufthansa, said it’s not clear whether the captain entered a code to try to get back into the cockpit when he returned, or whether Lubitz “put the lever on lock,” which would have prevented the code from working.

Lubitz doesn’t say anything in the audio recording during the flight’s descent. But the recording picks up his steady breathing, Robin said, with no sign that he had a heart attack or other medical issue.

In the final minutes of the flight, it also picks up the sounds of passengers screaming.

The big question: What happened inside the cockpit during those crucial last minutes?

The crash site

The plane went down in a rugged, sparsely populated part of the Alps. A local tourist official told French newspaper Liberation that the crash occurred on a particularly steep area of mountainside.

Helicopter crews found the airliner in pieces, none bigger than a small car, and human remains strewn for several hundred meters, authorities said.

German Foreign Minister Frank-Walter Steinmeier described it as “a picture of horror.”

Access to the crash site is reported to be difficult, with no roads leading to it. Authorities began to airlift some victims’ remains from the site Wednesday, a process complicated by freezing weather.

The big question: How long will it take for search teams to recover all the human remains and key parts of the aircraft wreckage?

The people on board

The captain of Flight 9525 had flown for Germanwings for more than 10 years and had more than 6,000 flight hours on the aircraft model, according to the airline.

Lubitz, the co-pilot, has been with Germanwings since September 2013 and had completed 630 hours of flight time, the airline’s media office said. Lubitz had trained at the Lufthansa flight training center in Bremen, Germany.

Germanwings hasn’t released details about the four other crew members.

Details have begun to emerge about some of the passengers on the plane, but officials have cautioned that there is still a degree of uncertainty at this point over who exactly was aboard.

Spain’s King Felipe VI has said “high numbers of Spaniards, Germans and Turks” were on the aircraft. The Germans included 16 students and two teachers from Joseph-Koenig Gymnasium, a school in the town of Haltern. Citizens of Argentina, Australia, Belgium, Britain, Colombia, the Netherlands and the United States have also been confirmed to have been on the plane by their national governments.

The big questions: Who was on board, and what are their stories?

The plane

The aircraft was a twin-engine Airbus A320, a model that is generally considered to be among the most reliable aircraft, aviation analyst David Soucie said.

According to information from the Aviation Safety Network accident database, there have been 55 incidents involving the A320, not including Tuesday’s crash.

In one of the most recent, in December, an Airbus A320 operated by AirAsia Indonesia crashed into the Java Sea en route from Surabaya, Indonesia, to Singapore. All 162 people on board Flight QZ8501 were killed.

The plane also made headlines in 2009 when Capt. Chesley “Sully” Sullenberger performed the “miracle on the Hudson” emergency landing on a river when his Airbus A320 collided with a flock of geese and lost thrust 2,700 feet over Manhattan.

The A320 that went down in the Alps was delivered to Lufthansa in 1991, Airbus said. It had clocked roughly 58,300 flight hours over the course of about 47,600 flights, according to the manufacturer. Germanwings said it was last checked Monday in Dusseldorf.

The big question: It seems unlikely at this point, but did a problem with the plane lead to its descent and eventual crash?

The investigation

Hundreds of French firefighters and police officers are involved in the recovery effort in the Alps. Searchers have so far retrieved the cockpit voice recorder, one of the plane’s two “black boxes,” said French Interior Minister Bernard Cazeneuve. Its exterior was damaged, but investigators were able to recover the audio file.

Officials said the outside frame of the second “black box” – the flight data recorder – has been found, though not the recorder itself. The flight data recorder stores a vast array of parameters about the aircraft.

The two devices are expected to be crucial in unraveling what led to the crash, though conclusive answers may not come quickly. Investigators typically spend months analyzing the recorders’ information.

Police searched Lubitz’s apartment on Thursday, seeking clues about his possible motive.

The big questions: When will the flight data recorder be found? What will the devices’ full contents reveal about events aboard the plane? And will investigators turn up anything to explain why the co-pilot may have decided to crash it?

CNN’s Eliott C. McLaughlin, Pierre Meilhan, Catherine E. Shoichet and Holly Yan contributed to this report.