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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 official said that audio from the mangled voice recorder of Germanwings Flight 9525 reveals the captain was locked out of the cockpit while the co-pilot appeared to make a deliberate attempt to destroy the plane.

And while it appears co-pilot Andreas Lubitz wanted to destroy the plane, there is “nothing to allow us to say that it was a terrorist attack,” Marseille prosecutor Brice Robin said. The German national was not on any terrorism list, he said.

Even as some blanks are starting to be filled — such as exactly who was on the flight and where investigators are in their probe — the news of a possibly deliberate crash raised many more questions, chief among them: Why was one of the plane’s pilots reportedly locked out of the cockpit before the crash?

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

The big question: If Lubitz brought the plane down, as it appears, why?

The final moments

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

A New York Times report Wednesday detailed one investigator’s account of audio from the plane’s cockpit voice recorder.

One of the plane’s pilots was locked out of the cockpit, and you can hear him trying to smash down the door in the audio recording, a senior military official involved in the investigation told the newspaper.

Lufthansa, the parent company of Germanwings, was looking into the report, spokesman Boris Ogursky said, saying the airline were avoiding speculation on the matter.

Early Thursday, Robin, the prosecutor, said audio from the voice recorder confirmed the earlier report.

CNN aviation analysts cautioned that it’s still unclear what could have been going on inside the cockpit. Possibilities, they said, range from a medical emergency that incapacitated the remaining pilot to something more nefarious, such as a suicide mission.

The fact the voice recorder picked up breathing in the cockpit, according to Robin, suggests the co-pilot was alive until impact, and the fact the cockpit door was placed in a locked position — preventing the other pilot from using a keypad and emergency code to get in — suggest he was locked out intentionally.

The big question: What happened in 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 all 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. It was badly damaged.

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.

The big questions: When will the flight data recorder be found? And what will the devices’ full contents reveal about events aboard the plane?

By Jethro Mullen

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